Margaret Cavendish attacked neuroscience—of the sort that Thomas Willis practiced—in order to save it. In her estimation, one really could study the brain and make sense of the mind. However, in doing so, natural philosophers would need to adopt new scientific methods, just as they would have to accept an image of the psyche radically at odds with human pride and the beliefs of dominant culture. Moreover, neuroscience would need to rethink some of its customary figures. Rather than personifying parts of the brain and thereby transforming the organ into a microcosm for tiny minds, Cavendish, at her most radical, likens the nervous system to a tool or prosthesis. As a result, studying the psyche would mean exploring its action or behavior rather than hunting for hidden cerebral mechanisms. For all its power, Cavendish’s critique and revisioning of neuroscience remained an outlier—despite some suggestive parallels to more mainstream work. Just a few years after Cavendish died, though, the philosopher John Locke launched an equally powerful, though altogether more influential, attack on the sort of neuroscience practiced by some of his contemporaries. Unlike Cavendish, Locke’s critique of brain science did not seek to save or amend the practice. On the contrary, Locke sought to overturn neuroscience entirely, so as to institute an entirely different method of accounting for the mind. Locke’s exploration of the psyche would pass over in silence the substance that made the mind work, and, in place of such speculations, it would stress and investigate first-person mental experience. In doing so, Locke, like Cavendish in this respect, came to treat the mind as an action, process, power, or faculty rather than a concrete entity perched in a physical place.1
Locke is important to my study in part because of this powerful and significant critique. Nervous fictions, I have argued, are a response to a gap in our knowledge. Because brain matter is so unlike the mental phenomena it somehow generates or supports, because a “Bowl of Curds” appears incapable of acts of imagination and will, one must use figure and fiction to make the substance of the brain resemble the stuff of mind: hence the animal spirits and other nervous matter become thinking, feeling, toiling entities in a bodily kingdom or state. Although a number of thinkers pointed to this gap, Locke gave the most complete and philosophically sophisticated account of it.2 In this respect, Locke’s work, by indexing the “clash” between matter and mind, also explicates early neuroscience’s reliance on the very figures and fictions it frequently disavows. Locke himself was clearly interested in, and suspicious of, the various nervous figures circulating in contemporary writings on the brain. He noted that, in the absence of a clear connection between nervous matter and mental properties, any neuroscientific account of the mind would become necessarily embroiled in linguistic or even literary matters. For example, in a passage from book 3 of An Essay Concerning Human Understanding, Locke maintains that certain confusions and questions arise not from a direct examination of the nervous system but from the “Imperfection of Words.” Locke recounts the meeting of a group of “very learned and ingenious Physicians” who “Question, whether any Liquor passed through the Filaments of the Nerves.”3 This seemingly pure scientific question, it turns out, cannot be settled by cutting into the nerves or examining them with a microscope. Instead, only by analyzing the words used to describe the brain can one hope to answer such questions:
The Debate having been managed a good while, by variety of Arguments on both sides, I (who had been used to suspect, that the greatest part of Disputes were more about the signification of Words, than a real difference in the Conception of Things) desired, That before they went any farther on in this Dispute, they would first examine, and establish amongst them, what the Word Liquor signified. . . . Upon Examination [they] found, that the signification of that Word, was not so settled and certain, as they had all imagined; but that each of them made it a sign of a different complex Idea. This made them perceive, that the Main of their Dispute was about the signification of that Term; and that they differed very little in their Opinions, concerning some fluid and subtile Matter, passing through the Conduits of the Nerves. (3.9.16)
Locke’s main point here is that we must agree on definitions, especially in matters related to the “complex ideas” we draw together to name the substance of things.4 But the example itself is telling, and is probably taken from real life—since Locke was intimately familiar with early neuroscience and its quarrels. Having acquainted ourselves with the same neuroscience, we can imagine some of the finer details of the debate Locke only sketches in this passage. We can assume that the “liquor” in question was related, in some capacity, to the animal spirits, which “flowed” through the nerves like water through a pipe after all. Furthermore, we can imagine that much of the debate concerned this liquor’s capacity to move through the body, its ability to carry information across the nerves, and even perhaps its power to produce ideas in the mind. As we will see, even after the proper linguistic terms are fixed in such cases, Locke would have insisted that such questions are intractable, since we cannot see how matter (or indeed any substance) relates to mind. In place of that knowledge, we will forever find ourselves wrangling over the definitions of words, caught in their ambiguities, making do with the imperfections of language.
It is somewhat odd, I realize, to treat Locke’s Essay as a text concerned with critiquing neuroscience, since so much of its bulk—and so much of the critical commentary upon it—bends toward a more constructive account of the understanding and its powers. But Locke’s positive account of the psyche arises in some measure from his criticism of neuroscience. For instance, one of the most important goals of the Essay is to justify and to defend the authority of first-person subjective experience. For Locke, we have privileged access to the contents of our psyches; we know our own minds better than anyone else ever could. To illuminate these aspects of the mind, Locke purposefully and knowingly employs metaphor. He famously figures the understanding as a self-enclosed space: a cabinet, room, or camera obscura, separated from the wider world, where ideas are bathed in a bright light and carefully cataloged. This image of the Lockean psyche should be familiar to most scholars of eighteenth-century thought—especially to those who have worked hard to analyze Locke’s metaphors of mind.5 What should be more surprising, however, is that this description of the mind’s powers results, at least in part, from a complicated and conflicted struggle with early neuroscience and its language. As Sean Silver has argued, Locke’s “enclosure” figures are adaptations of, and deviations from, earlier claims about the internal workings of the brain.6 Silver is ultimately interested in how these figures are grounded on a more primary worldly action (namely, Locke’s practices of archiving and collecting), but I want to extend this insight by explaining why Locke felt the need to move away from neuroscience by refiguring its language. By dissecting the organs of the understanding, neuroscience provides a third-person perspective on the mind. Locke’s claim is that such dissections can never outstrip the penetrating insights of one’s own introspective powers; studying the brain could never reveal facts about the conscious mind not already plainly evident to the thinking subject. Locke illustrates this dynamic by describing the mind as literally a power but only metaphorically a place.
As privileged witnesses to the ideas within the mind’s cabinet, then, we are the first and best observers of our understanding and its faculties. Nevertheless, having established Locke’s standard account of the mind, this chapter will conclude by considering an important exception to the Essay’s representation of the psyche—a moment that brings into even starker focus that work’s claims about the contrasts between first-person introspection and third-person reports on the nervous system. Locke maintains that, in the case of madness and the misassociation of ideas, the mind’s usual introspective powers fail. The mad cannot see that their minds have connected otherwise unlike ideas. To explain this malady, Locke attributes misassociated ideas to pathways carved into cerebral matter by animal spirits. In other words, to explicate madness, Locke writes his own nervous fiction. Significantly, Locke embraces neuroscience only when the mind cannot know itself.
It is implied early on in the Essay that there is a great deal at stake in circumventing neurophysiology. If the purpose of Locke’s work is, as he puts it, “to enquire into the Original, Certainty, and Extent of humane Knowledge” (1.1.2), then this project only gets off the ground once the weighty conjectures of neuroscience have been jettisoned from the Essay. In the opening pages of that work Locke explains, “I shall not at present meddle with the Physical Consideration of the Mind; or trouble my self to examine, wherein its Essence consists, or by what Motions of our Spirits, or Alterations of our Bodies, we come to any Sensation by our Organs, or any Ideas in our Understandings; and whether those Ideas do in their Formation, any, or all of them, depend upon Matter, or no. These are Speculations, which, however curious and entertaining, I shall decline, as lying out of my Way, in the Design I am now upon” (1.1.2). With questions concerning the physiological substratum of the psyche suppressed, the Essay can attend to its real subject: not the human brain seen as an obscure bodily organ somehow capable of producing ideas and sensations but rather the mind understood as an active and alive entity, one that grapples with ideas in the understanding and, through them, objects in the world. The above passage continues: “It shall suffice to my present Purpose, to consider the discerning Faculties of a Man, as they are employ’d about the Objects, which they have to do with: and I shall imagine I have not wholly misimploy’d my self in the Thoughts I shall have on this Occasion, if, in this Historical, plain Method, I can give any account of the Ways, whereby our Understandings come to attack those Notions of Things we have” (1.1.2).
To a reader weaned on a heady blend of materialist philosophy and cognitive neuroscience, the above passages could appear to enact a curious reversal. For one thing, the brain appears oddly insubstantial in the first quotation. Under Locke’s cautious gaze, the corporeal mind in its messy, vibrant reality—the brain pulsing with motion—dissolves into the stuff of rumor and speculation. In its place, the seemingly more abstract model of the understanding as it is “employ’d” with various mental objects—the psyche as a power rather than a piece of matter in motion—becomes a fitter object of study. Moreover, it is only by observing this nonphysiological model of mind, only by thinking of the mind as an agency rather than a physical structure that Locke can “set down any Measures of the Certainty of our Knowledge,” a task that relegates speculations on neurophysiology to “curious and entertaining” but ultimately idle or perhaps perplexing anatomy lessons.
That Locke consigns the “Physical Consideration of the Mind” to obscurity might surprise those critics who have read him as a kind of mistaken materialist.7 Nevertheless, claims like this are of a piece with the larger aims of his work and more specifically of a piece with the empirical nature of his philosophy. As Locke explains in the above passage, the Essay is written according to the rules of the “Historical, plain Method.” In the late seventeenth century, the phrase served as a term of art, one loosely adapted from the distinction between “natural philosophy” and “natural history.”8 Speaking generally: “natural philosophy” tended to systematize; it wanted to explain a multitude of phenomena by speculating upon first principles and fundamental causes. “Natural history” sought to observe available and evident facts without delving into hidden mechanisms. To borrow an example from Michael Ayers: Robert Boyle’s sincere conviction that the nature of reality could be explained by the motion and texture of invisible corpuscles is an example of “natural philosophy,” while Boyle’s insight into the inverse relationship of pressure and volume in a gas (i.e., “Boyle’s Law”) operates as a “natural history,” since this law merely records an observable phenomenon rather than tracing it to a more fundamental cause.9
For Locke’s Essay to read as a “history,” then, it too must confine itself to observing and cataloging the available facts of mental life. In practice, this means attending solely to the ideas that populate the conscious mind rather than guessing at their more fundamental material nature. As Locke puts it: “My present purpose being only to enquire into the Knowledge the Mind has of Things, by those Ideas, and Appearances, which God has fitted it to receive from them, and how the Mind comes by that Knowledge; rather than into their Causes, or manner of Production, I shall not, contrary to the Design of this Essay, set my self to enquire philosophically into the peculiar Constitution of Bodies, and the Configuration of Parts, whereby they have power to produce in us the Ideas of their sensible Qualities” (2.21.73). In other words, the precise nature of the mind’s ideas, their physical constitution, and the disposition of the organs that receive and retain them—all of these things lie outside the more limited purview of Locke’s “history.” They form a dark background to the starker images that draw the Essay’s attention: ideas as they are present and perceptible to the thinking mind. Ultimately the Essay, as one of its most careful readers pointed out, “is a history-book . . . of what passes in a man’s own mind.”10 But had Locke concerned himself with the “peculiar Constitution of Bodies, and the Configuration of Parts,” which, possibly, generate ideas in the psyche, had he considered the causes lurking behind or beneath “Ideas, and Appearances,” he would have produced an entirely different work. Not an “Historical, plain” account of the understanding, but something closer to the natural philosophical works I’ve surveyed in previous chapters.11
Importantly, although the Essay is commonly read as a work mainly invested in a constructive project—examining the understanding and its abilities—this constructive project also entails a critical dimension: namely, the cordoning off of impossible queries into the mind’s relation to matter. The Essay’s critical dimension is captured in a series of markedly figurative passages in its opening pages. Building on earlier admonitions against neurophysiology, these passages strengthen the case for abandoning epistemologically unsteady ground. We should, Locke writes, “prevail with the busy Mind of Man, to be more cautious in meddling with things exceeding its Comprehension; to stop, when it is at the utmost Extent of its Tether” (1.1.4). We ought to avoid acting like the servant “who would not attend his Business by Candle-light, to plead that he had not broad Sun-shine [since] The Candle, that is set up in us, shines bright enough for our Purposes” (1.1.5). We should take to heart the lesson of the sailor “who know[s] the length of his Line, though he cannot with it fathom all the depths of the Ocean. ’Tis well he knows, that it is long enough to reach the bottom, at such Places as are necessary to direct his Voyage, and caution him against running upon Shoals, that may ruin him” (1.1.6). And in an extraordinary moment where Locke draws on images of light and darkness in order to cast an epistemological warning in a dramatic chiaroscuro:
For I thought that the first Step towards satisfying several Enquiries, the Mind of Man was very apt to run into, was, to take a Survey of our own Understandings, examine our own Powers, and see to what Things they were adapted. Till that was done I suspected we began at the wrong end, and in vain sought for Satisfaction in a quiet and secure Possession of Truths, that most concern’d us, whilst we let loose our Thoughts into the vast Ocean of Being, as if all that boundless Extent, were the natural, and undoubted Possession of our Understandings, wherein there was nothing exempt from its Decisions, or that escaped its Comprehension. Whereas were the Capacities of our Understanding well considered, the Extent of our Knowledge once discovered, and the Horizon found, which sets the Bounds between the enlightned and dark Parts of Things; between what is, and what is not comprehensible by us, Men would perhaps with less scruple acquiesce in the avow’d Ignorance of the one, and imploy their Thoughts and Discourse, with more Advantage and Satisfaction in the other. (1.1.7)
Although this quotation does not mention the rejection of “Physical Consideration[s] of Mind” that worried Locke earlier in the Essay, it nevertheless repeats a similar lesson: one must hew away areas of ambiguity and uncertainty in order to clear a more stable and certain place for human understanding. More striking still, though, is the manner in which this passage imparts that message. For a moment, Locke ornaments his “historical, plain Method” with a series of images that turn mind and world into topographically distinct zones: spaces marked by their relative degrees of light and darkness. Thanks to these figures, the mind appears in the Essay as an illuminated, surveyed space, a site where our knowledge of things can be certain and evident (at least in principle). Beyond the confines of the psyche, however, stretches a vast, illimitable, and obscure waste, a realm where we at best reside in stupefied skepticism and at worst suffer in frustrated ignorance. The purpose of such images, of course, is to direct our attention toward the “enlightned . . . Parts of Things” (like the conscious mind), which we can perceive and know with a measure of certainty, and away from the “dark Parts” (metaphysical speculations into the underlying nature of reality as such), which only lead to darkness and doubt. In doing so, these images anticipate Locke’s favorite metaphor of mind: the cabinet or camera obscura, a figure that transforms the psyche into a well-lit and orderly space encompassed by outer darkness. Moreover, following through on the logic of such images, we appreciate Locke’s reluctance to engage in anatomical speculations into the substance of the brain. To study the mind as an agency, possessed with “discerning faculties” and “employ’d” with mental objects, is to catalog something comprehensible; to dissect the organ of thought means a plunge into the “boundless Extent” besetting our “quiet and secure” islands of truth. I’ll return to the bright confines of the conscious mind and to Locke’s constructive project and its metaphors later in this chapter. For now I want to cast some light on why Locke sees neuroscientific speculation as a kind of darkness besetting our best efforts to understand the psyche.
One reason Locke refuses to “meddle with the Physical Consideration of the Mind,” as we have seen, has to do with the Essay’s strategy for identifying and thereby heading off intractable questions about the ways in which matter relates to mind. But this strategy raises a series of questions in turn: Why did Locke insist that such questions are unanswerable in the first place? Why does the Essay contend that neurology, rather than aiding the “history” of the understanding, would only obscure such efforts? Why must Locke put matter out of mind before he can concentrate on the thoughts within the mind? Why does Locke insist that the body and brain’s effect on the mind stands in an epistemological blind spot? In order to answer these questions, it is important to consider some facts about Locke’s medical background before turning to a fuller consideration of the Essay’s rejection of natural philosophical accounts of the brain.
Locke’s attitude toward neuroscience is hardly born out of ignorance. As historians like Kenneth Dewhurst, Patrick Romanell, and Jonathan Walmsley have shown, Locke would have encountered new experimental work in natural philosophy as a medical student at Oxford.12 There he attended a series of lectures by Thomas Willis. Based on Locke’s own notes, we know that in these lectures Willis was developing some of the ideas that would reappear in his later books on the brain and nervous system.13 From Willis, Locke learned the basics of cerebral anatomy and, more significantly still, encountered accounts of madness, mania, and melancholy that attributed these mental states to the movements of animal spirits in the nervous system.14 But Locke left behind his interest in more speculative natural philosophical work with Oxford. Upon exiting its orbit, he gravitated toward the influence of another important physician: Thomas Sydenham.15 Thanks in part to Sydenham, Locke settled into the belief that one should not “meddle with the Physical Consideration of the Mind.” Seventeenth- and eighteenth-century thinkers continually describe Sydenham as a sort of anti-Willis.16 Unlike Willis, Sydenham never sought the underlying causes of the diseases he treated. Rather than hunt for animal spirits or invisible morbific matter, Sydenham simply tried to document, with utmost care and rigor, the manifest effects of his patients’ maladies. For Sydenham, medical science must remain content with these surface-level observations, since nature itself keeps deeper causal mechanisms beneath an impenetrable “veil.” As Sydenham explains, “It is in accordance with immutable laws, and by a scheme known to herself only, that Parent Nature accomplishes the generation of all things; and though many things she may bring forward from the abyss of cause into the open daylight of effect, it is in deepest darkness that she veils their essences, their constituent differentiae, their inherent natures.”17 Sydenham’s striking claim that “Parent Nature” “veils” the workings of bodies and matter in “deepest darkness”—only sometimes allowing an errant appearance to slip into “the open daylight of effect”—prefigures Locke’s troping on the epistemology of darkness and light, blindness and insight in the Essay itself. These echoes are unsurprising, since Locke clearly admired the older physician, even going so far as to name him in the Essay, along with Boyle, Newton, and Huygens, as one of the four “Master-Builders, whose mighty Designs, in advancing the Sciences, will leave lasting Monuments to the Admiration of Posterity” (Epistle). As Locke’s praise makes clear, he saw Sydenham’s epistemological modesty not only as a corrective to the overly speculative projects of some of his colleagues and teachers but also as a scientific advance in its own right.
Sydenham’s influence on Locke’s thought is perhaps most evident in one of the philosopher’s earliest pieces of writing, a short essay entitled “Anatomia.”18 The goal of this early essay is to demonstrate the inoperability of anatomy for practicing physicians, and at times it reads like a recasting of Margaret Cavendish’s similar attack on the art of dissection. Although the antiempiricist Cavendish and the archempiricist Locke are an admittedly odd pairing, the latter adduces many of the same problems with anatomy as the former. Locke doubts “that anatomie is like to afford any great improvement to the practise of physic.”19 A startling claim, to be sure, but one that Locke defends by insisting that anatomy can reveal only superficial aspects of the body and its organs: “All that Anatomie can doe is only to shew us the gross and sensible parts of the body, or the vapid and dead juices.”20 Locke certainly understands that dissections can dig up heretofore hidden organs. But in bringing these organs to light, the anatomist only uncovers a new mystery. It remains unclear, for instance, how the “the vapid and dead juices,” made evident by the dissector’s scalpel, generate the lively physiological effects that the practicing physician so carefully considers. In other words, anatomy tells us about the shape and structure of the body’s organs, but it cannot explain how those organs work. According to Locke, the real source of physiological function is simply not perceptible—no matter how penetrating the perspective:
Now it is certaine and beyond controversy that nature performs all her operations on the body by parts so minute and insensible that I thinke noe body will ever hope to pretend, even by the assistance of glasses or any other invention, to come to a sight of them. . . . For suppose any one shall have so sharp a knife and sight as to discover the secret and effective composure of any part could he make an occular demonstration that the pores of the parenchyma of the liver or kidneys were either round or square and that the parts of urin and gall separated in these parts were in size and figure answerable to those pores. I ask how this would at all direct him in the cure either of jaundice or stoppage of urin? What would this advantage his method or guide him to fit medecines?21
In this moment, Locke, sounding like Sydenham, draws a line between the “minute and insensible” parts that really make nature work and the “gross and sensible” organs witnessed in the anatomy theater. No matter how deep anatomists dive into the interior of the body, they can never arrive at the parts by which “nature performs all her operations,” since, in Sydenham’s terms, these parts cannot be dragged from “the abyss of cause into the open daylight of effect.” With this in mind, “Anatomia” asks physicians to focus on perceptible bodily effects, a project that hopefully will lead to more practical medical knowledge: “How [should the physician] regulate his dose, to mix his simples and to prescribe all in a due method? All this is only from history and the advantage of a diligent observation of these diseases, of their beginning, progress, and ways of cure.”22
Sydenham’s influence and the arguments of “Anatomia” are evident everywhere in the Essay. Indeed, much of Locke’s later work extends the medical pragmatism of these sources into a more general epistemology. For instance, Locke’s insistence that the Essay will provide a “Historical, plain” account of the mind rather than an explanation of the organs of thought follows naturally from Sydenham’s argument that physicians ought to focus on the manifest effects playing out before them rather than the latent causes of those appearances. Moreover, like Sydenham or “Anatomia,” Locke stresses throughout the Essay that we cannot glimpse the mechanisms that make bodies work. We simply cannot see or understand those “insensible Corpuscles, [that are] the active parts of Matter, and the great Instruments of Nature” (4.3.25). As Locke goes on to explain:
I doubt not but if we could discover the Figure, Size, Texture, and Motion of the minute Constituent parts of any two Bodies, we should know without Trial several of their Operations one upon another, as we do now the Properties of a Square, or a Triangle. Did we know the Mechanical affections of the Particles of Rhubarb, Hemlock, Opium, and a Man, as a Watchmaker does those of a Watch, whereby it performs its Operations, and of a File which by rubbing on them will alter the Figure of any of the Wheels, we should be able to tell before Hand, that Rhubarb will purge, Hemlock kill, and Opium make a Man sleep; as well as a Watch-maker can, that a little piece of Paper laid on the Balance, will keep the Watch from going, till it be removed. . . . But whilst we are destitute of Senses acute enough, to discover the minute Particles of Bodies, and to give us Ideas of their mechanical Affections, we must be content to be ignorant of their properties and ways of Operations; nor can we be assured about them any farther than some few Trials we make, are able to reach. (4.3.25)
In other words, physical bodies are not geometric figures or even watches. We know what parts make a triangle, and we know what gears make a watch tick. However, the same is not true of drugs (like rhubarb) or for that matter human beings. To be sure, we can see, in the “open daylight of effect,” that opium sedates and rhubarb heals. But how this occurs is hard to know, since the “minute Constituent parts” that make these effects possible are just out of view.
It becomes clear, then, that Locke’s skepticism—and some of the figurative language he uses to convey it—grows out of his early work in medicine and anatomy.23 Even more importantly, this same skepticism is at the root of his critique of neuroscience. For Locke, explaining mind-matter interaction is even more difficult than explaining how hemlock kills or opium sedates. In the latter cases, we can at least understand conceptually how one physical body (e.g., opium) might change or affect another physical thing (e.g., make a human body sleep). Although the invisible mechanisms that make such changes possible are invisible to us, we can imagine them easily enough: “That the size, figure and motion of one body should cause a change in . . . another body, is not beyond our conception; the separation of the parts of one body, upon the intrusion of another; and the change from rest to motion, upon impulse; these, and the like, seem to us to have some connexion one with another” (4.3.12). However, in the corresponding case of body’s relation to mind, we seem to be utterly in the dark. Not only are the mechanisms that make possible the interactions between mind and body invisible to us; more troubling still, we cannot even imagine how those mechanisms might work if we uncovered them, since in this instance there is no sensible “connexion” between the physical and the mental. Locke illustrates this point by underlining the distinction between what he calls the primary qualities (figure, texture, and bulk) actually inherent in physical bodies and the secondary qualities (smell, taste, color, etc.) that those bodies produce in our mind. As he notes, “our minds not being able to discover any connexion betwixt these primary qualities of bodies, and the sensations that are produced in us by them, we can never be able to establish certain and undoubted rules, or the consequence or co-existence of any secondary qualities, though we could discover the size, figure, or motion of those invisible parts, which immediately produce them” (4.3.12). In other words, in the case of mind-body interaction, there is a conceptual or epistemological gap forever separating cause from effect. Even if I could somehow see the “invisible parts” that cause a certain mental event—even if, for example, I could witness the atoms emanating from a flower interacting with precise neural mechanisms in my brain—I still would have no way of understanding how cause led to effect in this instance, since there is no evident relationship between these atoms, their interactions with my brain, and the resulting thought or sensation in my mind.
Another way of illustrating this tricky point is to note, as Locke does, that there is no “resemblance” between primary and secondary qualities (2.8.13). A violet may look blue and smell sweetly to us, but, strictly speaking, the particles composing the flower are colorless and odorless. Nevertheless, these same colorless, odorless particles, having made their way into our nervous system, produce entirely different sensations in the psyche, since a “violet, by the impulse of such insensible particles of matter . . . causes the ideas of the blue colour, and sweet scent of that flower to be produced in our minds” (2.8.13).24 Hence, the flower’s primary qualities (the blank corpuscles that compose it and that filter into the brain) are wholly unlike the secondary qualities generated in our mind (blue, sweet smelling, etc.). There is no “resemblance” between them. Moreover, because there is no clear connection linking primary and secondary qualities, we cannot explain, or make intelligible, why certain particles produce “blue” sensations while others create, say, “green” ones. Since there is nothing inherently “blue” or “green” to the invisible, colorless corpuscles that produce such colors, the connection between matter and mind in this instance must appear wholly arbitrary. Even if we were able to observe the precise corpuscles that create blue sensations, even if “we could discover the size, figure, or motion of those invisible parts” that create such ideas, it would still be unclear how they did so or, for that matter, why they generated a blue mental image instead of a green one or even a wholly different psychic event (like, say, the sense of euphoria). In this case, a gain in knowledge—we would observe matter actuating a specific mental event—plunges us into a deeper mystery: we would not know how matter accomplished this feat.25
What holds for sensation also holds for mind-matter interaction more generally. Just as we cannot know precisely how the body creates thoughts in the mind, we also cannot know how the mind manages to move the body:
As the Ideas of sensible secondary Qualities, which we have in our Minds, can, by us, be no way deduced from bodily Causes, nor any correspondence or connexion be found between them and those primary Qualities which (Experience shews us) produce them in us; so on the other side, the Operation of our Minds upon our Bodies is unconceivable. How any thought should produce a motion in Body is as remote from the nature of our Ideas, as how any Body should produce any Thought in the Mind. That it is so, if Experience did not convince us, the Consideration of the Things themselves would never be able, in the least, to discover to us. (4.3.28)
Importantly, Locke does not claim that matter bears no relation to thought or that the mind cannot bear along the body. On the contrary, as he notes here, “Experience shews us” that these relations exist. Again, what we cannot know is how these interactions occur. There is an evident link between certain physical events and certain psychic experiences. Anyone can observe, through sheer trial and error, that placing sugar on the tongue generates a sweet sensation in the psyche. But since there is no inherent resemblance between the chemical composition of sucrose and the taste of something sweet, we cannot provide a deeper, more thorough, and ultimately more satisfying explanation for this simple act. We can write down the formula for sucrose and describe, in exacting detail, the experience of consuming ice cream, but the transformation of matter to mind, in this instance, stands as a brute fact that seems to resist further explanation. And this will mean that neuroscience—as it was envisioned by natural philosophers like Locke’s teacher Thomas Willis—reaches, at a certain level of investigation, a real impasse. As Locke explains in a late work, “Impressions made on the retina by rays of light, I think I understand; and motions from thence continued to the brain may be conceived, and that these produce ideas in our minds, I am persuaded, but in a manner to me incomprehensible.”26 The chain of cause and effect that Locke traces here breaks at the most important link: the transformation of physical qualities (“motions . . . continued to the brain”) into mental images. It is Locke’s insistence that we cannot know the nature of this missing link—that we cannot observe matter becoming thought—that makes him one of the most thoroughgoing skeptics of neuroscience of his time.
I’ve belabored Locke’s sometimes difficult account of primary and secondary qualities in the hope of casting some light on his critique of neuroscience. In Locke’s thinking, the third-person perspective of neuroscience could never see beneath the veil separating the conscious mind from the rest of the world. Consider the admittedly paranoid example of the scientist probing into a brain while someone thinks. In this case, Locke would argue that the neuroscientist could not give the subject new or surprising information about what is happening in the thinker’s mind. At worst, the neuroscientist might reveal some superficial aspects of the brain; at best, the scientist might be able to tell us about the mechanisms that appear to cause certain mental events. But in neither instance, Locke would maintain, is the neuroscientist speaking about ideas “within” the mind. Only the conscious subject can do that. Locke illustrates this point by mocking Cartesian accounts of the neurophysiology of sight:
Those who tell us, that light is a great number of little globules, striking briskly on the bottom of the eye, speak more intelligibly than the Schools: but yet these words never so well understood, would make the idea, the word light stands for, no more known to a man that understands it not before, than if one should tell him, that light was nothing but a company of little tennis-balls, which fairies all day long struck with rackets against some men’s foreheads, whilst they passed by others. For granting this explication of the thing to be true; yet the idea of the cause of light, if we had it never so exact, would no more give us the idea of light itself, as it is such a particular perception in us, than the idea of the figure and motion of a sharp piece of steel, would give us the idea of that pain, which it is able to cause in us. For the cause of any sensation, and the sensation itself, in all the simple idea of one sense, are two ideas; and two ideas so different, and distant one from another, that no two can be more so. And therefore should Descartes’s globules strike never so long on the retina of a man, who was blind by a gutta serena, he would thereby never have any idea of light, or anything approaching to it, though he understood what little globules were, and what striking on another body was, never so well. (3.4.10)
In other words, the only way to know light is to see it, to experience it. A neuroscientific account of perception, at its most sophisticated and incisive, limns the physical mechanisms that cause ideas to appear before the mind’s eye. But the cause of an idea and the idea itself are entirely different things for Locke. In fact, the entire distinction between primary and secondary qualities hinges on this important detail. In the above example, the Cartesian neurophysiologist can tell us about the primary qualities involved in perception—the “number of little globules, striking briskly on the bottom of the eyes”—but this same scientist cannot explain how cause links to effect, how those invisible globules produce the phenomenal fact of light. Once again, there is no “resemblance” between the “little globules, striking briskly” and the actual experience of light. These are two different ideas, “and two ideas so different, and distant one from another, that no two can be more so.”
From one perspective, Locke’s skepticism must appear extraordinarily troubling. If “Experience shews us” that bodies cause ideas to appear in our minds, just as our minds cause bodies to move, and if Locke shows in turn that we cannot comprehend fully how such interactions occur, then it would seem there is a vast, boundless ocean of darkness present within the intimate confines of the self. How, we might ask, can I be certain of my ideas if I cannot see or understand the mechanisms that presumably produced them in me? How can I trust in my actions if I have no way of knowing how I made them happen in the first place—or even if I made them happen? We could avoid this dilemma, of course, by locating some “resemblance” or “connexion” between primary and secondary qualities, between the physical and the mental.27 Certain nervous fictions, I’ve argued, result precisely from such efforts. Through a series of (often disavowed) figures, some nervous fictions translate primary qualities into secondary ones. They make matter and mind resemble one another. With a nervous fiction, we could make the Cartesian account of sight more intelligible by transforming the animal spirits living in the optic nerves into loyal animals that can “spot” bright tennis balls and then, in a game of fetch, deliver them to the psyche seated in the pineal gland. Importantly, Locke’s insistent stress on the fundamental difference between primary and secondary qualities, matter and mind, demonstrates why such fictions often are written in the first place. Perhaps even more importantly, though, Locke himself opts for a different solution to this impasse, one that turns what appears to be a weakness into a strength.
We should be careful when stressing the limits of Locke’s account of human knowledge, since delving into the Essay’s skeptical corners can give the sense that Locke’s philosophy is a more modest, more cautiously superficial undertaking than it really is. In fact, the Essay’s self-imposed constraints in one area usually unloose possibilities in another. If Locke cuts off ideas at their roots, leaving accounts of their material and physical causes to wither away in dark conjectures, he does so in the hope of seeing them bloom all the more brightly in the mind. I want to suggest that Locke’s refusal to countenance physiology and neuroanatomy—his insistence that conjectures into the constitution of the brain must be kept out of the natural history of the understanding—is of a piece with his quest to gain certain knowledge of the mind by banishing uncertainty from its confines. To adapt some of Locke’s own figurative language, we can better survey human knowledge by first marking off its boundaries. Once we have cleared away areas of obscurity and confusion—like neuroscience or philosophical speculations into the substance of thought—we can set to work exploring territory that will reward our efforts. Hence, what Locke calls “the peculiar Constitution of Bodies, and the Configuration of Parts” that are the “Causes, or manner of Production” of the ideas that flicker before the inner eye must themselves remain out of sight and out of mind. Since no amount of work will bring certain aspects of the understanding to light, we are better off accounting for those elements of the psyche we can know with some measure of confidence.
In other words, Locke’s project is not simply critical or skeptical; it also has a constructive component that works to bolster the mind’s knowledge of itself. More specifically still, Locke’s critique of neuroscience effectively undergirds first-person experience, even as it undercuts the claims of brain anatomists who necessarily study the substance of thought from a third-person perspective. In fact, many of the elements we associate most strongly with Locke’s influential account of the psyche were put in place to serve, at least in part, as a bulwark against the sort of neurophysiological speculation that might diminish the authority of introspection. For Locke, no matter how well lit the anatomy theater, the mind, when reflecting on its own ideas and abilities, always sees things in a clearer, brighter, steadier, and more revealing light than an outside observer. Hence, much of the Essay represents an effort to adduce the power of introspection and to defend the reliability of its findings. In general, it does so by arguing that the psyche is transparent to itself (unlike the workings of other minds and physical bodies, both of which lie veiled from direct sight) and that thinking subjects possess privileged access to the workings of their own minds.28
For Locke, we are the first and best witnesses of the events transpiring within our own understanding; no one can tell us something about the mechanism of our mind that we didn’t know already or couldn’t confirm after a closer look. With some effort, we can examine our understanding at work, and, based on such observations, we can derive more general insights into the nature of mind. Much of the Essay jogs the reader through this process, though tellingly, given its emphasis on the importance of subjective experience, it never makes an absolutely objective claim about the psyche. Instead, Locke often asks the reader to look inward to validate the Essay’s observations. As Daniel Mishori notes, the Essay justifies its assertions through frequent “appeals” to its audience. Hence, Locke continually produces sentences like “Whether this be not so, I appeal to every one’s observation” (1.4.20); “But whether this be so, or no, I will not here determine, but appeal to every one’s own Experience” (2.8.5).29 Ultimately, in the Essay’s pages, a weakness—we can never know how matter impacts the mind, though we can see that it does—becomes a strength—we can never know how matter impacts the mind, because we already know everything there is to know about the mind. As we will see, Locke’s characterization of the mind as powerfully transparent to itself ironically generates some new obscurities, but for the moment I want to further detail its workings.
One reason introspective analysis outstrips other kinds of psychic exploration (such as the stripping away of flesh in neuroanatomy) is that, at least in principle, nothing escapes its view. For Locke, the psyche, seen in the light of self-reflection, has no unconscious, no dark corner or obscure vestibule where an idea or train of thought might lurk unseen and disregarded. Clearly, the same cannot be said of the mind revealed to neuroscience, since the physiologist necessarily encounters organs that evince no clear connection to the mental phenomena with which they relate. Locke’s well-known polemic against innate ideas nicely illustrates this point. According to innatist doctrine, gaining knowledge about certain universal principles means simply discovering ideas already somehow imprinted or engraved on the substance of thought at birth. Innatism implies that there are ideas within the mind that are nevertheless unknown to it (at least for a time). Locke rejects this logic. It is built, he insists, on linguistic confusion: “It seeming to me near a contradiction, to say, that there are truths imprinted on the soul, which it perceives or understands not: Imprinting, if it signify anything, being nothing else, but the making certain truths to be perceived. . . . To say a notion is imprinted on the mind, and yet at the same time to say, that the mind is ignorant of it, and never yet took notice of it, is to make this impression nothing. No proposition can be said to be in the mind, which it never yet knew, which it was never yet conscious of” (1.2.5). In this respect, Locke’s system closes the gap between having an idea somewhere within the mind and being able to perceive or know that idea: “For if these Words (to be in the Understanding) have any Propriety, they signify to be understood. So that, to be in the Understanding, and, not to be understood; to be in the Mind, and, never to be perceived, is all one, as to say, any thing is, and is not, in the Mind or Understanding” (1.2.5). Although the Essay’s primary task is to establish a “history” of the understanding through the use of introspection, that work sometimes engages in linguistic critique as a means of supporting and substantiating the findings of self-reflection. In this instance, a particular theory of mind would diminish introspection’s power, since the existence of innate ideas implies psychic entities that are “within” the mind but somehow unknown to it. But Locke can dismiss the innatists’ conception of mind, and thereby clear the way for his preferred model, by analyzing and teasing out the “propriety” of a common expression (“to be in the Understanding”). Importantly, to establish his constructive account of mind, Locke must unravel some of the seductive confusions inherent in our mental language.
To be sure, Locke also turns such language to his own advantage. For instance, one way he displays the mind’s transparency—and the resulting strength of its analytic powers—is through the calculated and knowing use of certain figures. As many of its readers have noted, the Essay’s ideal metaphor of mind is the cabinet or camera obscura (and not, as is sometimes assumed, the blank slate or tabula rasa).30 Although Locke’s writing is suffused with references to various “enclosures,” a passage at the outset of the Essay encapsulates the most significant features of this figure:
The Senses at first let in particular Ideas, and furnish the yet empty Cabinet: And the Mind by degrees growing familiar with some of them, they are lodged in the Memory, and Names got to them. Afterwards the Mind proceeding farther, abstracts them, and by Degrees learns the use of general Names. In this manner the Mind comes to be furnish’d with Ideas and Language, the Materials about which to exercise its discursive Faculty: And the use of Reason becomes daily more visible, as these Materials, that give it Employment, increase. . . . In Ideas thus got, the Mind discovers, That some agree, and others differ, probably as soon as it has any use of Memory; as soon as it is able, to retain and receive distinct Ideas. (1.2.15)
The cabinet conveys some important facts about mental life (according to Locke): that the mind receives all its ideas as particularized sense data from the external world; that these same ideas are retained, organized, and classified (“lodged in the Memory, and Names got to them”); and that “Reason” sets to work on the resulting “Materials” by tracing out their connections. More generally, though, the metaphor captures the mind’s aforementioned clarity. The understanding’s “contents” can be perceived quickly and cataloged precisely—a phenomenon highlighted by Locke’s imagery of the psyche as an orderly, well-lit space. In this respect, the cabinet serves as a sensible analogue to Locke’s more abstract views concerning the understanding and its introspective powers.
Locke is especially canny about metaphor’s power to convey the mind’s power. For example, he is adamant that we must not take his mind-as-enclosure metaphors literally. Should we imagine the mind as an actual cabinet or container, we will find ourselves facing a series of physiological and metaphysical questions that the Essay’s circumventions of neuroscience and innatism attempt to avoid. After all, if the mind physically stores ideas on some interior shelf, then what is the nature of that space? Where is it located precisely? What is it made out of? Likewise, what is the substance of the ideas stored there? How much space do they take up? What happens if we run out of storage? To steer clear of these impossible, vertiginous questions, Locke must cling to metaphor. And so when he discusses memory in the Essay, he makes a point of stressing the metaphorical nature of our customary mental language by unpacking the figurative logic at play in it. For Locke, when we say that an idea is stored “in” memory, we are simply speaking figuratively. More properly, when an idea is not directly perceived by the mind, it is literally not in the mind:
But our ideas being nothing, but actual perceptions in the mind, which cease to be anything, when there is no perception of them, this laying up of our ideas in the repository of the memory, signifies no more but this, that the mind has a power, in many cases, to revive perceptions, which it has once had, with this additional perception annexed to them, that it has had them before. And in this sense it is, that our ideas are said to be in our memories, when indeed, they are actually nowhere, but only there is an ability in the mind, when it will, to revive them again; and as it were paint them anew on itself. (2.10.2)
In other words, the mind is more like a power than a place. Or, rather, for Locke, the mind is literally a power but only figuratively a place. Strictly speaking, the understanding is the mind’s ability to perceive and consider its ideas, while memory denominates the mind’s capacity to revive previous perceptions and to see them as previous perceptions. Importantly, then, Locke insists that the mind must not be understood as a structure or locality, as a site with length, breadth, and depth, or as space with an inside and an out. When we speak of the mind in these terms, we are speaking in figures. More specifically, we are using concrete, spatial imagery to capture more abstract mental capacities. It is easier to imagine memory as a repository than a power. But Locke is clear that the understanding and its faculties are actions or processes rather than locations or things.
This point is worth stressing, since it elucidates an often misunderstood aspect of the Lockean mind, even as it illustrates one way the Essay twists away from neuroscience. More specifically, Locke’s purposeful metaphors of mind distance his thinking from his neuroscientific rivals and predecessors. Arguably, earlier neuroscientists concern themselves with metaphors passed off as literal structures in the body. As we have seen, animal spirits are personifications taken for particulate matter; the “Cartesian theater” is a synecdoche—the whole organism’s perception of its environment is internally redoubled in a part of its body—turned into an actual cerebral porthole. Locke implicitly forges his own account of mind by critiquing these literalizations and by reactivating their figurative origins. If the Essay were to cast the mind as a physical space, then it would reproduce the neurophysiological systems of Descartes or Willis. After all, both Descartes and Willis install a “cabinet” or “theater” at the center of the brain, imagining that spot as a place where the soul or psyche can sit and judge the ideas delivered to it by the nerves and spirits. To be sure, Locke’s account of the mind shares many qualities with Descartes’s or Willis’s. All three thinkers see the psyche as a singular, unified, and self-transparent entity. But Locke faults Descartes and Willis for grounding the mind in the necessarily obscure mechanisms of the brain and body. If the mind perches in the pineal gland, then it faces a mystery: the transformation of physical stimuli, the stuff of primary qualities, into mental phenomena or secondary qualities. Locke’s gambit, paradoxical as it may seem, is that he can save and even strengthen aspects of the Cartesian or Willisian psyche by treating it as a faculty (something we do) rather than as a substance (something we have). Put differently, Locke believes he can rescue the systems of Descartes and Willis by turning their forgotten figures back into evident tropes. In this respect, his frequent references to cabinets, theaters, and enclosed spaces are an artifact of the older neurophysiological systems his work draws upon in order to renew.31 Unlike Descartes or Willis, who took these enclosures as actual structures in the brain, Locke treats them as metaphors for the more intangible but ultimately clearer and more certain workings of the mind. For Locke, the mind is a concrete entity not because it is located in a specific spot in space or time (such as the brain or pineal gland). Instead, it is concrete because we experience its workings in the most direct manner possible. That is to say, the Lockean mind is anchored to the ineluctable fact of our conscious experience—not to the pineal gland.
Like Cavendish before him, then, Locke’s figures transform the psyche from a microcosm populated by ideational elements into an agency, action, or tool for thinking. Although unlike Cavendish, Locke, still in thrall to certain aspects of Descartes’s or Willis’s work, stresses this transformation so as to grant the mind immediate, intuitive, and inviolable knowledge of its ideas. For Locke, the move from mental space to thinking agency accords the understanding greater power. According to the Essay, “to be in the Understanding” just means “to be understood.” To knowingly play with one of Locke’s knowing metaphors, it is as if the understanding’s cabinet is bathed in a remorseless light. As a result, the mind’s knowledge is clear as day, since knowledge itself is “nothing but the perception of the connexion and agreement, or disagreement and repugnancy of any of our ideas” (4.1.2). In Locke’s epistemology, we gain certain knowledge once the psyche glimpses corresponding ideational connections. To deny such connections would be to stare at the sun without seeing something bright. Put differently, we have no choice in knowledge since its attainment happens in the blink of an (internal) eye: “When the agreement of any two ideas appears to our minds, whether immediately, or by the assistance of reason, I can no more refuse to perceive, no more avoid knowing it, than I can avoid seeing those objects, which I turn my eyes to, and look on in daylight” (4.20.16).
But as many of the Essay’s critics, culminating with Richard Rorty, have noted, the clarifying powers of the Lockean mind arguably derive from a deeper confusion: namely, the conflation of passive sense impression with the active attainment of knowledge.32 Rorty reminds us that usually knowledge requires justification. When we come to believe certain things, we give or receive reasons that justify that knowledge.33 If we cannot provide a justification for why we take something to be the case, then we are not talking about knowledge but about some other kind of mental state. However, by “model[ing] knowing on seeing” (as Rorty puts it), Locke makes it seem as if, having perceived the connection of some ideas, certain knowledge requires no further justification.34 In fact, for Locke, even if we were pressed for an additional reason or justification for why or how we know something, we only could ever say that, in the daylight of understanding, this is the way things look to us. On the one hand, Locke’s confusion of knowing and seeing ensures greater certitude in our mental lives. Thanks to the understanding’s perceptive powers, the Lockean mind can trade a variety of knowledge that requires reason, explanation, and justification (and so is always open to refutation or further refinement) for a variety that is inherently seen, known, and certain. On the other hand, as a result, the Lockean mind becomes so powerful that the subject becomes, paradoxically, passive in the face of certain knowledge. On a certain level of certainty, one cannot “refuse to perceive” and thereby “avoid knowing.” Knowledge is something that happens to the subject rather than something one achieves or attains.
The problems with the Lockean approach to epistemology are manifold. But let me point to one that even Locke’s keenest critics tend to miss—and, stranger still, one that Locke himself anticipated. If knowledge is “nothing but the perception” of ideational “agreements,” then what happens if someone sees a connection between two otherwise disparate sets of ideas? For example, what if someone sees an “agreement” between the idea of their body and the idea of glass (2.11.13)? As a result, the person becomes convinced that he is a literal “glassy essence,” a fragile body that must be handled with care. Because this glassy person sees the connection between these ideas with as much intuitive certainty as he does the light of the sun, there is no way to convince him that he holds a false belief. After all, his knowledge derives not from argument or evidence but from the same introspective powers that make knowledge seem clearly evident. Curiously, the very power of the understanding, its ability to know things with absolute certainty, has robbed the mind of some agency. The glassy person cannot reason or talk his way out of this false belief. Locke’s name for this odd phenomenon—the perceived connection of an ideational agreement where anyone else would see disparity—is madness. And given the phenomenon, we might imagine that Locke would soften his stance on epistemology. Instead, he returns to neuroscience.
For Locke, madness makes the mind doubly deranged. First, madness confuses the understanding’s ideas. It occurs, Locke explains, when “Ideas that in themselves are not at all of kin, come to be so united in some Mens Minds, that ’tis very hard to separate them” (2.33.5). Second, madness creates a kind of internal blindness. It makes the mind unable to perceive and thereby rectify the aforementioned ideational confusions. In this respect, madness is the “most dangerous” of “the Errors in the World . . . since so far as it obtains, it hinders Men from seeing and examining” their thoughts (2.33.18). Indeed, while we can observe madness in others, we cannot see this malady at work in ourselves. As a result, reason, argument, and evidence have no effect on the false associations of madness. But if the mind, taken as a power rather than a literal place, is inherently known to itself, then why are the mad incapable of “seeing and examining” their own ideas? As John P. Wright points out, it was only in the fourth edition of the Essay that Locke landed on something like an explanation—though one that departed radically from the stated aims of that work.35 In this edition, Locke argued that madness is rooted in the body and brain.36 More specifically, he contended that madness’s propensity both to confuse and to blind the understanding results from “Trains of Motion in the Animal Spirits,” which carve “a smooth Path” into the brain (2.33.6).
It’s hard to know how to understand this new theory of madness, since it seems to challenge Locke’s carefully argued agnosticism on neuroscientific matters. Strictly speaking, it is a contradiction: Locke cannot state, on the one hand, that correlating mental qualities with an underlying physical substance is impossible in principle while, on the other hand, doing just that in this new explanation of madness and misassociation. One might soften the contradiction by arguing that Locke, in this instance, is merely pointing to the physical cause of madness rather than offering a complete account of its origins—just as the Cartesian posits “little globules” that cause the idea of light without fully explaining how these particles light up the mind. But this reading cannot make the theory fully cohere with the rest of the Essay, since it must ignore the specificity and thoroughness of Locke’s neurophysiological account of madness. While it certainly traces a causal dynamic, Locke’s chapter on the association of ideas insists on drawing a very precise connection between specific events in the brain (the animal spirits’ carved pathways) and particular experiences in the mind (confused ideas that cannot be seen as such). In doing so, it exceeds the limits laid down by the rest of the Essay.
Locke’s embrace of neurophysiology cannot be dismissed so easily, then. After all, it parallels other important moments in the Essay. Consider that, in a text obsessed with detailing the ways in which we are in the dark about mind-matter interaction, Locke’s chapter on madness, which describes a psyche defined by its blindness to the physical forces shaping its thoughts, appears at least thematically in accord with the rest of the Essay and its anxieties. As we have seen, Locke tries to allay that anxiety by turning an epistemological blind spot into a greater source of clarity. For Locke, we cannot know how matter impacts mind, because we already know everything we can about the mind. But madness returns Locke to the skeptical energies held at bay in the Essay and guarded against by its account of transparent, inherently authoritative minds. Faced with the phenomenon of psyches that clearly work differently than those the Essay usually surveys—minds that connect otherwise disconnected ideas—Locke again entertains some of the troubling implications of neuroscience.
We can detect something odd in this new chapter on the association of ideas and madness—something that will drive Locke to embrace, once again, neurophysiology. We’ll remember that, for Locke, one of the most important tasks of the Essay is to establish the authority of first-person experience: I know my psyche better than another can know it; the neuroscientist, or some other third-person viewer, cannot tell me something about my mind that I don’t already perceive. Hence, the Essay’s “appealing” methodology: an insistence that the reader introspect in order to confirm Locke’s more general claims about the understanding. The chapter on the association of ideas embraces a similar methodology but enacts a curious twist. It appeals to a common experience that the reader presumably could confirm through an act of introspection, even as it radically undercuts that same power: “There is scarce any one that does not observe something that seems odd to him, and is in it self really Extravagant in the Opinions, Reasonings, and Actions of other Men. The least flaw of this kind, if at all different from his own, every one is quick-sighted enough to espie in another, and will by the Authority of Reason forwardly condemn, though he be guilty of much greater Unreasonableness in his own Tenets and Conduct, which he never perceives, and will very hardly, if at all be convinced of” (2.33.1). The passage flickers between a reminder of the understanding’s powers of observation and a warning about the frightening blind spot within the same faculty. On the one hand, madness should be evident to anyone who bothers to observe the incongruous actions and beliefs of humanity; on the other hand, the same keen-eyed observer of madness in others is necessarily ignorant of it in himself. The result is a strange sort of universality, where everyone shares the conviction that he or she is the one sane person in a mad world. Most troubling of all, then, madness indicates, in contrast to Locke’s obsessive insistence in nearly every other chapter of the Essay, that the mind has a dark corner, a place that cannot be seen through introspection but that can be glimpsed by others. As Cathy Caruth points out, Locke’s “argument implicitly suggests that there is at least one type of self-reflection that does not take the simple form of reflective transparency, and involves a sort of interpretive self-distance.”37 It’s a point made starkly by the chapter’s continual references to imagery of light and dark, to moments of blindness and insight. Of the mad, Locke will note that “there must be something that blinds their Understandings, and makes them not see the falshood of what they embrace for real Truth. That which thus captivates their Reasons, and leads Men of Sincerity blindfold from common Sense, will, when examin’d, be found to be what we are speaking of” (2.33.18). Likewise, the specific examples Locke adduces of madness often reference similar dynamics, as if the particular cases were repeating, on a more exemplary level, the deeper mechanisms fueling them. Locke will complain of the association of goblins and darkness or point to the example of a “cured” madman who, strangely, persists in at least one insane belief: he cannot bear to see the physician who healed him (2.33.10; 2.33.14).
At least at first glance, madness appears as a wholly anomalous phenomenon for Locke’s philosophy, one that he will need neuroscience, the remainder or excess of his system, to account for. The recourse to neurophysiological explanations of madness seems to surprise even Locke himself. According to the Essay, education, custom, and in particular early childhood trauma play a key role in mental derangement.38 But importantly the Essay also insists that culture only partly explains the peculiar madness of association. There is another, deeper mechanism at work here:
This sort of Unreasonable [i.e., madness] is usually imputed to Education and Prejudice, and for the most part truly enough, though that reaches not the bottom of the Disease, nor shews distinctly enough whence it rises, or wherein it lies. Education is often rightly assigned for the Cause, and Prejudice is a good general Name for the thing it self: But yet, I think, he ought to look a little farther who would trace this sort of Madness to the root it springs from, and so explain it, as to shew whence this flaw has its Original in very sober and rational Minds, and wherein it consists. (2.33.3)
Attributing “prejudice” to education and custom, then, cannot exhaust all the symptoms of madness. In particular, it cannot explain how otherwise reasonable minds become mad. In order to examine the root cause of this malady, Locke looks beyond kinds of thinking and considers instead the physiology of thought. After introducing a distinction between “natural” and “unnatural” association—between those trains of ideas that have a “natural Correspondence and Connexion” and those that have been joined together irrationally—Locke goes on to trace the latter to the animal spirits within the brain:
This strong Combination of Ideas, not ally’d by Nature, the Mind makes in it self either voluntarily, or by chance, and hence it comes in different Men to be very different, according to their different Inclination, Educations, Interests, etc. Custom settles habits of Thinking in the Understanding, as well as of Determining in the Will, and of Motions in the Body; all which seems to be but Trains of Motion in the Animal Spirits, which once set a going continue in the same steps they have been used to, which often treading are worn into a smooth path, and Motion in it becomes easy and as it were Natural. As far as we can comprehend Thinking, thus Ideas seem to be produced in our Minds; or if they are not, this may serve to explain their following one another in an habitual train, when once they are put into that tract, as well as it does to explain such Motions of the Body. (2.33.6)
In other words, education and custom are at the origin of false association, but they are only the first link in a chain of causes that leads irrevocably to madness. On the other end of the chain is the body and more specifically the animal spirits in the brain and nervous system—precisely the sort of explanation Locke scoffs at elsewhere in the Essay. Hence, in madness, culture and physiology become confused. Society or some chance trauma suggests a wrong association of ideas; habitual thinking reproduces and repeats this misassociation; the movement of animal spirits then inscribes this disorder directly onto the flesh. In this respect, education, custom, and habit literally carve certain ways of thinking into the body. The animal spirits in turn ensure that superstition and whim become part of brain anatomy. This explains why the mind sees connections, or misassociations, between disparate ideas. It also clarifies why the association of ideas “hinders Men from seeing and examining” their errors and misconceptions. Thanks to the repetition of custom and habit, absurd ways of thinking thus become “easy and as it were Natural.” In this case, “ease” distracts the mind from the hard work of pursuing truth; it naturalizes and normalizes prejudice and error.39 The random flux of matter rather than rational thinking “determin[es] the Will.” Moreover, when made mad, the mind’s ideas have a mind of their own: “When this Combination [i.e., false association] is settled and whilst it lasts, it is not in the power of Reason to help us, and relieve us from the Effects of it. Ideas in our Minds, when they are there, will operate according to their Natures and Circumstances” (2.33.13).
It’s worth underlining just how markedly un-Lockean these passages are. For one thing, Locke abandons his medical pragmatism when he deals with madness and the association of ideas. We’ll remember that Locke, following the example of Thomas Sydenham, abjures dark speculations into the hidden workings of the body in order to focus on a more empirical “natural history” of disease. Since we can never observe directly the “minute and insensible” bodies that produce disease, Locke and Sydenham recommend that physicians attend to the “superficial” effects of their patients’ maladies and abandon the hunt for hidden and obscure causes. Yet, faced with the disease of madness, Locke himself gazes directly into what Sydenham called “the abyss of cause.” Education and prejudice—the most readily observable sources of madness—“reach not bottom of the Disease, nor shew distinctly enough whence it rises” (2.33.3). In order to understand false association, one must “trace this sort of Madness to the root it springs from,” namely, to the body and brain (2.33.3).40
This deviation from Locke’s official stance on medical matters points to yet another divergence in his thought. In the Essay, Locke’s medical pragmatism shades into a more general call for a cautious empiricism: just as we cannot observe how cause leads to effect in disease, we also cannot observe how matter in the body makes thoughts in the mind. Given our ignorance, Locke implores us to study the mind through introspection and conceptual analysis. Simply by observing the understanding at work, we can know everything there is to know about the psyche. No deeper form of explanation is required. We cannot look “behind” or “beneath” mental activity to some more interior cerebral realm, since the mind, properly conceived, is more like an activity or a power than a structure or a substance. Only by misrecognizing Locke’s metaphors of mind as literal descriptions might we be tempted to dig into the understanding’s insides. Nevertheless, in the case of madness, Locke himself takes on precisely this task. Consider once more his ultimate explanation of how the madness of misassociation works. He begins, first, explaining things from the mental, or first-person point of view, by pointing to the workings of various faculties: “Custom settles habits of Thinking in the Understanding, as well as of Determining in the Will, and of Motions in the Body.” But with a semicolon, Locke flips into an account of the material events underlying these acts of mind: “all which seems to be but Trains of Motion in the Animal Spirits, which once set a going continue in the same steps they have been used to, which often treading are worn into a smooth path, and Motion in it becomes easy and as it were Natural.” Here Locke treats the mind as a physical structure, and once he does so, he naturally considers its surface effects and interior depths. That is to say, he subtly but surely begins to narrate not the “plain history of the understanding” but a different sort of story: a “double life legend,” wherein mental dispositions (such as habits of thought) are matched by mechanical events (the “tread” of animal spirits in the brain) happening behind the scenes.
In this respect, Locke’s analysis of madness represents a return to the neuroscience he rejected early in his career. If this is the case, though, is it fair to say that Locke is writing his own nervous fiction in the mode of Descartes or Willis? On the one hand, it certainly seems so. To be sure, in his account of the association of ideas, Locke’s nervous fiction is more subtle than many I’ve surveyed in this book—though it operates according to a similar logic. In this instance, Locke connects the mental (the “habitual train” of ideas marching through the mad psyche) and the physical (the “trains of motion in the animal spirits,” which create “smooth path[s]” in the brain). As should already be evident, Locke papers over any disparity between mind and matter in this passage by drawing on metaphorical imagery that casts both cognitive processes and neurophysiological events as implacable “trains.” Because the Essay frequently describes the understanding as a faculty that can compare and contrast a succession of discrete ideas, it makes sense for Locke to speak of ideational “trains.” In madness, for instance, “trains” of certain ideas become “habitual”; the madman might think of his body and then, through force of habit, think next of glass—never noticing, along the way, the evident “disagreement” of these images. But “train” is a term that could describe any set of particular entities arrayed in rows or series, and Locke is happy to play with the wide referential ambit of the word since it allows him to parallel thoughtful trains with otherwise distinct “trains of motion” in the nerves and brain. In Locke’s language, the two “trains” become similar, when in fact their “cars” are made of fundamentally different substances (ideas, motions) and are pushed along by distinct dynamics (the perceived “agreement” or “disagreement” of ideas, the drive of physical force). Effectively, Locke’s nervous fiction makes it seem as if the same cognitive process unfolding “within” the understanding also plays out upon the brain.
Nevertheless, Locke’s nervous fiction radically departs from the form in certain respects. Usually a nervous fiction employs personification. In Willis’s writing, for example, madness results from mad animal spirits. Although Locke comes close to treating the spirits as internal homunculi—one can almost witness their feet when the Essay references their “steps” and “treading”—he resists these more extravagant figures. Locke’s animal spirits behave as they are supposed to. They are not thoughtful or willful creatures. Instead, they are physical particles knocked about by external forces. The paths they carve into the brain are the effect of sheer blind erosion rather than the result of more purposeful actions. Another subtle difference in Locke’s nervous fiction explains this switch. Most nervous fictions graft a mental quality onto a physical mechanism; that is, they begin by observing some mental characteristic and then claim to find that characteristic at work on a deeper level of neuroanatomy. Locke’s own account of madness reverses this sequence. Rather than making matter look like mind, he makes mind look like matter. The mind’s cognitive “trains” are steered into cerebral tracks: physical structures that determine the course of irrational thought. Because the mad are, in a certain sense, “thoughtless”—they cannot see, know, or control the connection of their ideas—Locke is happy to compare their style of thinking to thoughtless physical mechanisms. If nervous fictions normally attempt to cross the gap between mind and matter through figurative means, then in this moment we witness something like the limits of the form: an almost literal account of the brain and nerves that works to mechanize the mind.
I’ve argued that Locke articulates one of the clearest critiques of early neuroscience. He helps us limn the explanatory gap that troubles most efforts to reduce mind to matter, and in doing so, he also exposes the cause of early natural philosophy’s continual turn to fiction and figuration in representations of the brain’s relationship to the mind. In addition to his well-known description of the mind as a cabinet or internalized space, Locke’s critical account of neuroscience represents one of his most important contributions to eighteenth-century thought. Nevertheless, it’s worth noting that Locke’s chapter on madness and the association of ideas, which contains one of his clearest forays into a neuroscience he otherwise rejected, became influential in its own right later in the eighteenth century.41 It is a historical irony that although Locke undoubtedly understood his comments on madness as a brief digression from his constructive account of the psyche, later thinkers were happy to develop his hints into a full-fledged philosophy of mind—and some were even happy to extend Locke’s neurophysiological speculations. More curious still, in at least a few instances, we find the same conflict playing out in the work of Locke’s followers as we perceive in Locke himself. Just as Locke pushes away neuroscience only then to return to it in his theories of madness, Locke’s inheritors evince a similar pattern. Throughout the eighteenth century, the same thinkers who diagnose the problems inherent to neuroscience—thereby identifying neurophysiological speculation as a kind of fiction—are happy to indulge in such speculation when they must describe the mind operating in an unthinking or confused state. Apparently only after the mind’s introspective faculties have failed can neuroscience’s perceptive powers take over.
Before discussing the more complex responses to Locke, let me say a few words about the simpler ones. To be sure, the eighteenth century witnessed a spectrum of reactions to Locke’s theories of association. Many of the writers who followed Locke—or at least followed him on the association of ideas—happily ignored his chapter’s musings on cerebral traces and treading animal spirits. Just as it was possible to turn Locke’s more limited interpretation of misreasoning into a general account of all human cognition, it was also possible to jettison the neurophysiological aspects of his theory. In this respect, references to the brain or nerves in the writing of early associationists like John Gay or Francis Hutcheson are few and trivial. For such writers, one could explain the psyche’s associational laws without making reference to a deeper level of neural matter determining such laws.
On the other end of the spectrum, though, are thinkers like David Hartley, who, writing nearly fifty years after Locke, not only borrowed from the earlier philosopher’s associationist psychology but also expanded upon the neurophysiological traces of the Essay. For Hartley, the mind’s propensity to associate ideas was due to vibrations at play in the brain. According to Hartley, “external Objects impress vibratory Motions upon the medullary Substance of the Nerves and Brain.”42 In most cases, these cerebral vibrations fade away in time like the thrum of a musical string. But if the impressions are strong enough, or if they happen repeatedly, the brain stores them away for later use. Furthermore, if a series of vibrations tend to occur at the same time, then the brain links together those vibrations and their corresponding ideas: “Any sensations A, B, C, &c. by being associated with one another a sufficient Number of Times, get such a Power over the corresponding Ideas a, b, c, &c. that any one of the Sensations A, when impressed alone, shall be able to excite in the Mind b, c, &c.”43 As should be clear from even these brief comments, Hartley’s conception of the brain and nervous system is wholly different from Locke’s understanding of these organs. Hartley’s system represents the full flowering of the Newtonian physiology I discussed in the previous chapter and will return to later in the book.
But between these two extremes we find accounts of associationism that are, like Locke’s own, vexed in their use of neuroscience. For instance, both Joseph Addison and David Hume take up some of Locke’s ideas in order to extend them into other realms. Addison applies certain aspects of associationism to aesthetic concerns when he notes that sometimes small reminders can recall previous encounters with beautiful, novel, or sublime landscapes,44 while Hume famously makes association the mind’s principal mode of cognition (rather than an unfortunate side effect of madness).45 Although Addison and Hume move beyond Locke in manifold ways, they nevertheless retain some of his epistemological modesty. Addison insists that we cannot trace the pleasures of the imagination to some underlying cause “because we know neither the Nature of an Idea, nor the Substance of a Human Soul.”46 “For want of such a Light,” Addison continues, we must be content with studying the mind’s surface effects. That is, we can only reflect “on those Operations of the Soul that are most agreeable, and to range, under their proper Heads, what is pleasing or displeasing to the Mind.”47 Hume undertakes similar efforts at establishing clear boundaries in explorations of the mind’s ideas. At the outset of the Treatise, he points to “a kind of attraction” drawing together the psyche’s simple ideas. “Its effects,” he notes, “are every where conspicuous; but as to its causes, they are mostly unknown, and must be resolved into original qualities of human nature, which I pretend not to explain.”48 Still, he readily admits that readers will want to know the deeper nature of the forces shaping their mental worlds. The task of a “true philosopher,” however, is “to restrain the intemperate desire of searching into causes.”49 According to Hume, “having established any doctrine upon a sufficient number of experiments,” we must “rest contented with that,” since a “farther examination would lead . . . into obscure and uncertain speculations.”50 In Addison’s and Hume’s work, then, we detect the influence of Locke’s warnings about diving into dark speculations concerning the fundamental nature of thinking substance. In their own ways, Addison and Hume call for a “plain, Historical” account of the mind, one that focuses on what the psyche can perceive in the clear light of self-reflection. They also share in the metaphorical imagery the Essay established, with depth (the realm of those causes “behind” or “beneath” conscious experience) cast as unknown, uncertain, obscure and surface (the provenance of the psyche’s evident effects) made to seem tractable and apparent.
However, like Locke before them, Addison and Hume also turn to neuroscience when they need it to explain certain aspects of the mind. In one Spectator paper on the pleasures of imagination, Addison finds himself wondering why remembering a single piece of a beautiful landscape often reproduces—without any real recollective work on the part of the thinking subject—an image of the entire scene within the mind. To account for this odd phenomenon, he alludes to a “Cartesian” theory that attributes a “Sett of Ideas” to an underlying “Sett of Traces” within the brain.51 As is typical of such “Cartesian” speculations, Addison’s offers a split perspective, sometimes speaking of the mind and other times considering underlying matter. When “Ideas arise in the Imagination,” “a flow of Animal Spirits” is dispatched through certain cerebral pathways: “These Spirits, in the violence of their Motion, run not only into the Trace, to which they were more particularly directed, but into several of those that lie about it. By this means they awaken other Ideas of the same Sett, which immediately determine a new Dispatch of Spirits, that in the same manner open other Neighbouring Traces, till at last the whole Sett of them is blown up, and the whole Prospect or Garden flourishes in the Imagination.”52 Although it is unclear what agency actuates the animal spirits’ initial movement, once present in the brain, these cerebral entities behave as any material particle would. They are propelled into all available brain traces, thereby actuating new ideas. Because an unconscious, autonomic process is being described here—one thinks of a leaf and then an entire garden blooms in the brain all on its own—Addison doesn’t need the animal spirits to take on more mindful behaviors, and so he can avoid personifying these cerebral agents. As in Locke’s account of association, a mindless process merits an account of mindless mechanisms.
Hume also finds himself threading the cerebral labyrinth on occasion. Like Locke, Hume needs the resources of neuroscience only when he must explicate a mind that misbehaves. As Marina Frasca-Spada notes, “On practically every occasion that Hume mentions animal spirits and brain traces it is in connection with errors and the mechanisms underlying them.”53 Incredibly, Hume himself insists that, at any moment in the Treatise, he could draw on such terms. “It would have been easy,” he explains, “to have made an imaginary dissection of the brain, and have shewn, why upon our conception of any idea, the animal spirits run into all the contiguous traces, and rouze up the other ideas, that are related to it.”54 But significantly he only performs this “imaginary dissection” when he explains why the mind, sometimes searching for one idea, instead dredges up another. This occurs, Hume insists, because the animal spirits’ “motion is seldom direct, and naturally turns a little to the one side or the other; for this reason, the animal spirits, falling into the contiguous traces, present other related ideas in lieu of that, which the mind desired at first to survey.”55 In other words, when the mind is not fully in command of its thoughts, brain matter, figured here as entirely unthinking, can take over. Again: it’s hard to know if we should call such accounts nervous fictions. On the one hand, they cast the brain as an internal microcosm where cerebral processes originate trains of thought. On the other hand, as we’ve also seen, this space and the elements within it remain resolutely mechanical. The animal spirits are never granted minds of their own, and as a result they tumble through furrows and tracks rather than purposefully navigating a cerebral labyrinth. Typically, nervous fictions work to link mind and matter. But in these cases, mind is simply reduced to matter. At a certain extreme, then, nervous fictions simply become materialist accounts of the mind—a fact that perhaps explains the period’s odd reluctance to indulge in this form when accounting for the association of ideas.