Organic bodies, as we have already begun to see, are, taken separately, never individuals, since they are always mutually interdependent with countless other organic bodies, just like the termite and the protozoa in its stomach that enable it to digest wood, or the rhinoceros and the bird on its back. The real individuals are the simple substances, and their individuality consists precisely in their status as worlds apart, or as causally self-sufficient automata. Organic bodies, while not individuals in any rigorous sense, may be picked out as this or that organic body in view of their functional unity and activity, but to be an individual in any robust metaphysical sense necessarily involves a unifying soul or soul-like principle, insofar as the matter of the organic body is constantly in flux.
Living, perceiving, corporeal substances consist in part in organic bodies, and so do all of the infinitely many substances implicated in these bodies. In seeking to determine what considerations led Leibniz to propose such a picture of substance, some of the first and most visible road signs appear to point in the direction of the empirical life sciences. In this chapter, we will first be focusing on the biographical and historical context of Leibniz’s model of organic bodies. In particular, we will look at the relation of Leibniz’s model of organic body to the empirical life sciences of his era, especially to the seventeenth century’s discovery of the ubiquity of subvisible living creatures as well as its growing awareness of the deep interdependence of all living entities. We will conclude with a consideration of the legacy of the model of individuality developed by Leibniz in today’s philosophy of biology.
As we have seen, for Leibniz every corporeal individual has parts, at least in its bodily component, and whatever consists in parts is divisible. The divisibility of an individual suggests, if only for purely etymological reasons, that the “individual” in question is not really one at all. Since whatever is divisible is a physically cohesive bit of matter, and, vice versa, since any cohesive bit of matter is at least in principle divisible, philosophers throughout history have argued that inanimate portions of matter most definitely cannot be considered true individuals. There is an inherent vagueness to the identity of any entity that is thrown together from sundry parts: rocks and tables, no less than heaps and balding heads, fall victim to sorites paradoxes, as there is no crucial piece of them such that after its removal they cease to be the thing they previously were. But if physical cohesion is the only criterion of individuation we can find for each of the two halves of a split chunk of organic body, should we then conclude that such a body is on a par with rocks and tables, and that each of the two halves are not really individuals at all?
For Leibniz, again, a corporeal substance consists in the union of a soul or entelechy and an organic body. Insofar as a finger is divisible from the organic body, the bodily component of a corporeal substance is divisible. But because the corporeal substance as a whole is not divided as a result of this amputation, because the amputation does not yield two physically separate instances of the same substance, and because the rest of the body, without the finger, does not cease to be the body of the individual of which it had been the body prior to the amputation, it would be incorrect in Leibniz’s view to claim that the substance itself has been divided. Analysis of organic bodies will yield neither homoeomeries nor atoms, but only more, smaller entities of the same kind as the initial object of analysis. Analysis of the organic body of a corporeal substance will yield only more corporeal substances. These, being corporeal substances just like the corporeal substance whose body they compose, will in turn also yield only more corporeal substances upon further analysis.
Why does Leibniz insist, contrary to Aristotle and to the atomists alike, on the impossibility of analysis of organic bodies into ultimate constituents? Part of the answer is deeply theoretical and a priori: for Leibniz, the world consists exhaustively in individuals. All that there are in the world are individual substances and their properties. Such individuals must be unique and have unity and identity over time. But individuals also contain other individuals (as the bodies of animals include worms or germs). Not only that, an individual corporeal substance (or, as Leibniz puts it in his most mature period, an individual “animal”) may properly be seen as being the substance it is in virtue of the fact that it involves these other individuals, organized in a hierarchical structure, nested one within another. This structure of nested individuals,1 however, must have substantial unity, that is, it must be united as one substance. As Leibniz writes to Burchard De Volder in 1703:
Although I said that a substance, even though corporeal, contains an infinity of machines, at the same time, I think that we must add that a substance constitutes one machine composed of them, and furthermore, that it is activated by one entelechy, without which there would be no principle of true unity in it.2
As opposed to artificial machines, as we saw in the previous chapter, a machine of nature for Leibniz entails an infinity of machines that form a single unit. This very distinction seems to indicate that there is an intrinsic connection between Leibniz’s notion of organic unity and this unity’s nested structure. As we have seen, the unity of a composite substance derives from the entelechy or dominant monad, which is at once its singular source of activity as well as its source of unity.
On Leibniz’s model of nested individuality, an individual substance is a union of an active entelechy or substantial form animating and organizing its organic body.3 In order to qualify as an individual substance, an entity requires true unity, and it is the organic, living unities, such as animals and plants, that constitute the paradigmatic examples of such substantial unities, in contrast with aggregates, such as rocks, lakes, flocks, and armies. Nonetheless, there is no part of an aggregate that is not entirely composed of entities conceived on the model of plants and animals. Animals and plants are individual substances that are composed of other such animals and plants, or similar organic unities, nested in them, and the animals entailed in an individual substance are complete individual substances that have a similar structure. They are not mere “parts of the substance but are immediately required for it.”4 The structure of nested individual substances, finally, involves a hierarchy of dominating and dominated substances, which is constitutive of the nature of living individuality.
To the extent that, for Leibniz, a corporeal substance is a stratified structure of infinitely many substances, his model suggests a radical break from the implicit formula of at least the unitist strain of the Aristotelian metaphysical tradition: “one body, one substance.” Such a break comes at a price, however. Leibniz’s model is in tension with some deeply rooted intuitions about individuality that are still alive today. In addition to the counterintuitive claim that many substances may be implicated in one greater or more encompassing substance, the notion of nested individuality seems to conflict with the traditional logical and grammatical characterization of an individual substance as that which is “neither said of a subject nor in a subject.”5 Aristotle’s formulation of what is essential to individuals would remain extremely influential, as it provides not merely an articulation of our common intuitions about individuality but also underlies some of our scientific notions concerning biological individuality.
In our current notion of biological individuality, biological individuals are identified with multicellular organisms—organisms in our sense, that is, not “organism” in Leibniz’s sense—and for this reason groups or parts of organisms are excluded from the ranks of the complete individuals.6 Both of the extreme cases, of genes and groups—which have been proposed not only as biological individuals but also, within the context of evolutionary biology, as units of selection—illustrate our intuitive response. It is hard to accept a group, say, a beehive, as a single individual, because it already consists in multiple organisms, to wit, bees, notwithstanding the fact that they can only multiply as a group, and notwithstanding the fact that there is a cohesion to the beehive as a whole that seems to justify calling it at least as much a unity as, say, the body of a large mammal (which, of course, requires the cooperation of a number of internal, microscopic symbionts in order to survive). Ludwig Feuerbach, interestingly, understood Leibniz’s theory of nested individuality on the model of the beehive. He writes in an 1837 study of Leibniz’s philosophy:
The body, which the monads bring together and hold together, is the beehive. The dominant monad is the queen or mother bee. The bees do not live in such a loose connection as the beasts of a herd; they constitute one whole; every individual bee is to be seen as just one member of this organism, having only a partial life [Theilleben], a particular function, like an organ in my body. At the same time, though, every bee is an individual in itself, a particular being that stands on its own legs. Just as the self-standing bees constitute one organism, in the same way we must also understand the monads, as they together constitute a body.7
Feuerbach’s comparison is suggestive, but strictly speaking it does not get things quite right. In Leibniz’s model, the mammal’s body, considered apart from the soul that unifies it into one corporeal substance, truly is like the beehive in that it is constituted out of subordinate individuals and in that it has, on its own, at least a sort of mechanical, functional unity. Individuals function as constituents of other individuals. According to Leibniz’s model, a given component of my body may be considered simultaneously as a complete individual and as a constituent or requisite of me.
Leibniz thus significantly alters the unitist model of the structure of the bodily component of a form-body compound. For him, as for Aristotle, existing individuals are characterized by their inherent entelechy or inherent principle of activity, which also gives them their unity and identity over time. Leibniz holds that each component of an individual substance has its own entelechy while being included in a body and subordinated to the entelechy of the whole corporeal substance. In contrast with prevailing ideas about bodily parasites in the ancient world, which took them as by definition abnormal, for Leibniz nestedness, or the presence of other individual corporeal substances in the constitution of corporeal substances, is precisely constitutive of individuality. What it is to be such and such individual corporeal substance is a question that is inseparable from that corporeal substance’s constitution out of other corporeal substances.
It is worth noting that the notion of nestedness described here is not foreign to current biology, even if commonsense ideas about biological individuals remain out of step with biological reality. Thus David Hull writes that “the first thing a biologist does in arguing that an entity can or cannot function as a unit of selection is to argue that it is or is not an individual.”8 “One body, one substance (or individual)” remains, nonetheless, a formula that has proven difficult to abandon in the philosophy of biology, perhaps because it is ultimately a formula that is very much in keeping with common sense, whereas empirical data about the structure of the living world, ever since the development of microscopy in the seventeenth century, have consistently demanded that we reject our commonsense understanding of that world. To cite one example of the endurance of the ancient formula, in metaphysics if not in biology, Peter van Inwagen writes:
It cannot be that the activities of the xs [i.e., the proper parts of a living being] constitute at one and the same time two lives. Lives are, in fact, [such that] only in certain special cases can two lives overlap: Only in certain special cases can there be xs and ys such that the activity of the xs constitutes a life and the activity of the ys constitutes a life and the xs are not identical with the ys and, for some zs, the zs are among both the xs and the ys.9
According to Leibniz, in contrast, as, mutatis mutandis, for the Latin pluralists before him, full individuals may at the same time function as subordinate constituents of greater individuals. There is for him no difficulty in conceding levels of individuality. For much of history the true individual was thought to be simply the “organism” (again, in a non-Leibnizian sense), and still today many who are involved in the debate concerning the true unit of selection in evolution continue to search for the unique level—whether the gene, the organism, the group, or the ecosystem—at which the true individual may be said to reside. As Hull also notes there is a close relation between the question of what may count as units of selection in evolutionary theory and what may count as a biological individual.10 He defines a biological “individual” as “an entity which is systematically the target of selection,”11 while a “unit of selection” is in turn generally defined by three criteria: phenotypic variance, fitness variance, and heritability of characters relating to fitness.12
To sum up, one important consequence of Leibniz’s understanding of a living being is that it implies a plural notion of individuality, which recognizes infinitely many levels of it within any corporeal substance. Another consequence is that in this model, individuality and unity are defined through activity, not primarily through spatiotemporal cohesiveness. There must always be some spatiotemporally cohesive, organic body parts or other, but what makes these parts the parts of this or that organic body is by no means this cohesion, since, after all, the cohesion never lasts very long, and in the end an organic body has no more of it than, as Leibniz enjoys telling us, does a fountain or river. Thus what enables us to talk about an individual organic body is its functional unity and activity, which in the end may be traced back to the inherence of a soul, but which may also be discerned, as Leibniz shows in, for example, the Corpus hominis, looking at the body only qua machine.
Leibniz’s attempt to do justice to the new empirical life sciences in his metaphysics of corporeal substance has been noted by many scholars, but often this is in the course of denying the importance of corporeal substance to Leibniz’s central philosophical concerns. Corporeal substance is treated as a “concession” to the empirical, which itself is beneath the usual high standard of Leibniz’s rationalist philosophy. Robert Adams, who maintains that Leibniz never took seriously the ultimate reality of corporeal substances, no more in his middle period than in his later period, asserts that Leibniz’s “talk of ‘corporeal substance’ was usually rooted in an interest in accommodating within his system at least verbally, and if possible more than verbally, a common sense or traditional realism about bodies.”14 Catherine Wilson writes similarly, though with different basic commitments guiding her claim: “That the animalcula were so obviously and interestingly ‘there’ was perhaps . . . one reason why Leibniz would not embrace a pure phenomenalism in which it would have made no literal sense to speak of a natural world within which a subject was situated.”15 Wilson asserts that Leibniz could easily have bought consistency for his metaphysics by giving up his insistence that a soul cannot exist independently of a body, and suggests that while for Descartes the most important activity of a soul is abstract thought, for Aristotle and for Leibniz it is perception and motive power. The split in Leibniz’s thought between phenomenalism and realism comes, as Jacques Roger similarly explains, from Leibniz’s desire to “rendre compte du réel,” to avoid excessive detachment from the subject matter of natural science:
In [biology, Leibniz] remained above all an attentive spectator, a philosopher striving to ground the close relations between his thought and the most recent discoveries of contemporary thinkers. The study of these relations is not less useful, for it allows us to see that Leibniz’s thought does not intend to remain in the abstractions of ontology or pure logic, but rather seeks to take the real into account, including that reality represented by the living being, which is so resistant to abstract generalization.16
Perhaps a more interesting question, one that Adams, Wilson, and Roger do not ask here, is not so much why Leibniz would be willing to compromise his most pristine and abstract philosophy of simple substances, but rather why, in his effort to “take account of the real,” Leibniz does it precisely in this way. His philosophy of body cannot simply be a regression or concession to traditional realism, for there is nothing at all traditional about the account of bodies that Leibniz gives. The theory of nested individuality outlined in the previous sction is in many respects a radically new account,17 and Leibniz surely would not have gone to the trouble of developing it if organic bodies were not something he took seriously. He would have settled with an available model rather than elaborating a revolutionary one. Even if in the end the body is just part of the “story of science,” why does Leibniz give the particular account of it he does, as organic body? We have already argued that this has something to do with empirical science and also with Leibniz’s philosophical project of accounting for “material plasticity” or capacity for growth and motion in terms of the vegetative structure alone of the body. Let us turn now to some of the historical factors, particularly those stemming from empirical science, involved in Leibniz’s model of organic body.
Before the development of microscopy in the seventeenth century, parasitism was the variety of symbiosis, understood broadly as mutual occupation of the same body, which garnered the most attention from philosophers and physicians alike. On Aristotle’s view, for example, the presence of a worm in another animal body never bodes well. In the Historia animalium he mentions dogs driven by the insatiable hunger intestinal parasites cause to eat the standing corn,18 thereby inconveniencing the humans who would have eaten it, and fish in which “an intestinal worm, which develops in them at the time of the dog-star, makes them surface and weakens them: and having come to the surface they are destroyed by the heat.”19 He also describes the misfortunes of worm-ridden sponges.20 In the De generatione animalium, Aristotle mentions on at least three occasions the problem of parasitism in various animals (sponges, fish, dogs). Nowhere does he mention any sort of nonharmful mutualism or symbiosis between individual organisms.
In addition to Aristotle, many authors of ancient medical texts devote a great deal of attention to the causes and treatment of sickness due to intestinal and other parasites. Alexander of Tralles, for instance, a sixth-century Byzantine physician, writes that “the wide worms ultimately reach such a size that they extend throughout the entire intestine. . . . They grow when food enters and the undigested juices turn into rottenness.” Alexander recommends taking attar of roses in order to kill the worms and “purge them through the stool, reawakening the lost and weakened appetite.” He warns against trying to starve the parasites to death, since often, out of a shortage of food, “the worms eat right through the entrails, so that they can be seen to come through the skin.”21 Two creatures sharing the same body was considered abnormal and pathological, and, correlatively, the paradigmatic animals were the more or less discrete and autonomous ones, the ones that any nonexpert could enumerate without worrying about overlap. Cases of overlap in the same body or the same spatiotemporally cohesive organic assemblage were consequently taken as exceptions to the norm and certainly as more characteristic of odd or exotic species than of horses, men, and other such paradigmatic instances of substance.
Jack Wilson has argued that throughout the history of philosophy and science, metazoan animals such as horses and men, and easily individuated plants such as oak trees, have served as the paradigm cases of biological individual.22 This is in large part true, but in glossing over a very long history Wilson misses the sharp shift that takes place in the seventeenth century in scientific thinking about the paradigmatic or most basic corporeal substances in nature: from the horses and men of Aristotle’s Categories, science shifted its attention to “worms,” just as the broader scientific and popular culture of the era became fascinated with the plurality of worlds both too large and too small to be detected through unaided perception. In the seventeenth century, for the first time, the idea began to circulate that smaller organic bodies in the body of a larger one are not just inhabitants but indeed constituents of the body in which they were found. This shift of focus from macro- to microorganisms compelled researchers to search for microscopic “worms”—a term that in its early modern usage, again, referred to all small, creeping or slithering creatures, and sometimes even insects—at the source of a wide variety of biological phenomena, the investigation of which the ancients could only carry as far as their natural faculty of vision would allow. These phenomena included the explanation of epidemics, which in the seventeenth century came to be understood as the result of the presence of microscopic contagion in the environment rather than in terms of the Hippocratic miasma theory. As we will discuss in the following chapter, they also included the phenomena of generation.
How does Leibniz’s own work reflect these broad changes? According to what has been called Leibniz’s “worlds-within-worlds” doctrine, as stated in the notes he took on a letter of Michelangelo Fardella dating from 1690,
there are substances everywhere in matter, just as points are everywhere in a line. . . . [J]ust as there is no portion of a line in which there is not an infinite number of points, there is no portion of matter which does not contain an infinite number of substances.23
This thesis, as Leibniz explains in his Primae veritates of 1689, is tantamount to the claim that “there are no atoms,” and thus that “there is no body so small that it is not actually subdivided.”24 Matter has no ultimate, indivisible parts any more than does a line.25 The earliest description of this position in explicit terms of “worlds” is found in the 1671 text titled On Primary Matter: “Matter is actually divided into an infinity of parts. There is in any body whatever an infinity of creatures.”26 For the moment, the precise nature of these “creatures” remains unspecified, but this will change in later elaborations of the same doctrine. Leibniz writes to Antoine Arnauld some years later that
the human is a being endowed with a true unity given him by his soul, in spite of the fact that the mass of his body is divided into organs, ducts, humors, and spirits and that these parts are undoubtedly filled with an infinity of other corporeal substances endowed with their own Entelechies.27
In the same correspondence, Leibniz cautiously suggests that what he has in mind when speaking of “worlds” are in fact living beings:
And since matter is infinitely divisible, no portion can be designated so small that it does not contain animated bodies, or at least bodies endowed with a primitive Entelechy or (if you permit me to use the concept of life so generally), with a vital principle; in short, corporeal substances, of all of which one can say in general that they are living.28
In 1699, Leibniz writes to De Volder similarly: “You ask further if an animate body has its own entelechies distinct from the soul. I reply that it has innumerable such entelechies, since it consists in turn of parts each of which is animated.”29 And in a striking passage in a letter to Bernoulli of the same year: “I confess that there are parts in cheese in which there appear to be no worms. But what prevents there from being other smaller worms or plants in those parts in turn, or other organic things that are sui generis, and so in infinitum, so that there would be nothing in the cheese free from such things?”30 In correspondence with Des Bosses, Leibniz quite often makes explicit the nature of the substances making up the organic bodies of larger substances: “you [Des Bosses], ask (for example) whether the soul of a worm existing in the body of a human is a substantial part of the human body, or whether it is rather a bare requisite, and that not by metaphysical necessity but only because it is required in the course of nature.”31
There is perhaps a difficult interpretative question that must be asked in looking at passages such as these. Sometimes, philosophers choose their examples because they are interested in just that kind of thing that is serving as the exemplar. But sometimes philosophers choose their examples because they require a neutral object that will help them to make a point that could be made about many different sorts of object: thus in the classroom philosophers regularly talk about chalk, not because they are interested in chalk, but because they need a simple (and, ideally, uninteresting and therefore nondistracting) example of a physical object. Similarly, when Leibniz mentions armies as an example of aggregates, he has no intention of contributing to military science; he is doing metaphysics, and armies happen to provide a useful illustration that could have been made just as well by reference to something else. The usual approach to Leibniz’s mention of worms in passages such as those we’ve just looked at has been to take them as neutral examples of a much broader interest, a model of the structure of corporeal individuals that could be instantiated by any number of kinds of being. But there is good reason to think that these statements about worms are more narrowly vermicological than Leibniz’s statements about armies are military-scientific. To the extent that the category of “worm” is understood by Leibniz to include any small corporeal being, and given that the model of nested individuality, or the doctrine of worlds-within-worlds, is explicitly intended to account for the structure of organic bodies (which may in the end be traceable back to the perceptions of simple substances but is not for that reason deprived of its intrinsic interest, for Leibniz), it follows that Leibniz’s worms are not intersubstitutable with other, equally useful examples. Worms really are the elements of bodies.
Leibniz’s response to the question he has attributed to Des Bosses in the last passage cited above is that the worm is indeed a substantial part of the human body, and, moreover, at the same time it is itself a substance, that is, a dominant monad with an organic body: “Some worm can be a part of my body and be subject to my dominant monad, and the same worm can have other animalcules in its body subject to its dominant monad.”32 Again, while Jack Wilson is in general correct in holding that the commonsense view of biological individuation has remained the same from Aristotle to the present day, Leibniz’s own work is indicative of a noteworthy shift in the latter half of the seventeenth century, if not in common sense, then at least in natural science and philosophy, toward widespread acknowledgment of the cohabitation of multiple individuals in larger organic bodies. This shift is also evidenced by the publication of treatises such as Nicolas Andry’s De la génération des vers dans le corps de l’homme (On the Generation of Worms in the Human Body) of 1700 (two years after Leibniz first identifies the corporeal substances in his body as “worms” in a letter to Bernoulli33). Andry does not, as had Alexander in his treatise on worms, offer advice on how to purge the worms, thereby returning to one’s normal state, but instead describes with amazement and pleasure the utter normalcy of their presence there. Catherine Wilson has described Andry’s work as primarily concerned with the misfortunes wrought by worms. Indeed, Andry does blame them for “vomiting, pain, bloating, sweating, convulsions, stagger, and muteness.”34 The overall aim of Andry’s book, however, is not to indict worms as villains, but rather to give an account of their important role in the composition, and in all of the processes, including but not limited to the painful and unfortunate ones, of the human body.
Similarly, the microscopist Nicolaas Hartsoeker, in a 1699 letter to Andry (from the same period as Hartsoeker’s correspondence with Leibniz), describes the normalcy of worms in the human body without mentioning their role in disease: “[Worms] have been found in all the parts of the human body.” Curiously, he reports that they have “even [been found] in the pineal gland, if what I have been assured of is true.”35 The intention is evidently to challenge traditional ideas about what constitutes the individual human being, going so far as to allow “worms” into that very organ that for Descartes had stood out as the great hope for fixing the individual human’s soul-based identity to a particular body part. If the pineal gland, like the rest of the body, turns out to be inhabited by creatures nonidentical with the self, indeed not even human, then its candidacy for the role of “seat of the soul” must clearly be called into question. With Andry, Hartsoeker, and Leibniz, the pineal gland is now the seat of many souls.
This process of normalization of symbiosis that is evident in Hartsoeker, Andry, and Leibniz at the end of the seventeenth century would continue into the eighteenth century. In Friedrich Lesser’s 1738 treatise Insectotheologia, a work that could fairly be said to belong to the genre of theodicy, the particular aim being to demonstrate that the apparent nastiness of mosquitoes and worms is not evidence against the perfection of God, the author notes with joy that it is further evidence of “the wisdom and power of God” that “man, the most noble of animals, is a world in which a multitude of insects live. . . . Our intestines are not more free of them than those of other animals. . . . Our whole body is nothing more, so to speak, than a butcher-shop, providing meat to an infinity of insects.”36 While Lesser concedes that this newly discovered role of the human body may under some circumstances be painful and unhealthy, ultimately he sees it as yet more evidence of God’s goodness, power, and wisdom. He continues:
How admirable is the providence of God! It has seen not only to the housing of man; but with an infinite wisdom it has also seen to that of all the other species of animals that there are on earth. They are all destitute of reason; nonetheless there is not one of them that is not endowed with a natural instinct, which brings it to inhabit the environment proper to it, and where it will find the nourishment that best suits it.37
Charles Bonnet, similarly, in his Considérations sur les corps organisés of 1762, will offer this vivid description: “The mite, like the elephant, the louse, like the ostrich, the vinegar eel, like the whale, are nothing but composites of animals; all of their liquors bring them forth: all of their vessels are seeded by them.”38
These accounts seem not to bemoan, but only to acknowledge and sometimes to celebrate the ubiquity of microorganisms and their presence in the bodies of macroorganisms. Leibniz’s account to Des Bosses of the presence of worms in his body is a very good example of this celebration. Another is Balthasar de Monconys’s remarkably prescient claim in 1647 that “nature has provided all animals and plants with an infinity of minute invisible insects, which are for the purpose of sucking and drawing out the corruption and impurities of living things.”39
Leibniz, in sum, fits very well in this tradition of theodicy-style reasoning that extends from Monconys to Bonnet about the ultimate goodness and normalcy of symbiosis. In a striking passage from the Entretien de Philarète et d’Ariste of around 1713, he explicitly claims a bodily dependence on “worms” gnawing at him: “What does it matter if the worm that gnaws at me is within me or outside of me? Am I any less dependent upon it? Only incorporeal substances are created independent of every other created substance.”40 Leibniz’s purpose is apparently to distinguish between the independence of monads considered apart from the corporeal world that results from their perceptions, on the one hand, and on the other the essential dependence of bodies upon other bodies for their continued existence. Considered as an immaterial monad or soul, Leibniz is indeed independent; everything would remain the same if it were only God and he existing in the universe. But considered as a corporeal substance—that is, as the immaterial monad that is Leibniz’s soul together with an organic body—Leibniz’s identity is essentially dependent upon the coexistence and mutual representation of infinitely many monads, all with their own organic bodies and all implicated in the functional unity of Leibniz’s body-machine. The body is the body it is in consequence of this infinite aggregation of mutually gnawing worms. If this sounds macabre, let us recall that a similar point was made—albeit without worms—in the Corpus hominis of the early 1680s, in which Leibniz identified the animal as a quasi-perpetual-motion machine that perpetually requires something external to it—namely, food—for its continued existence. It is of the nature of a living body to be in constant interchange with what lies beyond it. It is in this sense that the body has no more diachronic fixity than a fountain, and it is also in this sense that Leibniz can understand the body mechanically without having to resort to the power of the soul to hold it together. Properly speaking, the body is not held together at all; it “gnaws” constantly at the outside world and is gnawed at in turn, and the only thing that gives it identity over time is the perpetual incorporation of newly gnawed bodily matter into the same functional unity in which recently cast-off bodily matter had previously been implicated.
The body’s constant interchange with the world outside of it seems to have come into Leibniz’s philosophy via the chemical and iatrochemical conception of nutrition as a sort of fermentation. From where, though, did the idea come of worms gnawing from within? Again, some of the first signs seem to point to the developing empirical research program of microscopy. The neologism “microscope” was invented by Johan Faber in 1625, but Galileo had been looking through his “fly-glass” since at least 1610. Catherine Wilson reports that the Jesuit priest Athanasius Kircher, who exercised tremendous influence on the young Leibniz, had microscopes in his possession by 1634.41 Kircher had devoted an entire chapter to the study of nature by means of the microscope in his 1646 work Ars magna lucis et umbrae. In his Scrutinium physico-medicum of 1658, the Jesuit scientist presents a theory of putrefaction that is based, as he claims, on decades of microscopic research.
Kircher is known to have been among the figures who influenced Leibniz during the latter’s most formative years.42 In the Dissertatio de arte combinatoria of 1666—a work, incidentally, in which the young Leibniz espouses the earliest version of his worlds-within-worlds doctrine—Leibniz describes Kircher as “immortal” and anticipates that in his forthcoming book, the Ars magna sciendi, the Jesuit will “penetrate into the heart of things.”43 Leibniz’s familiarity with Kircher’s work in general during this period makes likely a familiarity with Kircher’s microscopic interests in particular. In a 1669 letter to Thomasius, in turn, Leibniz, drawing on his knowledge of Robert Hooke’s microscopic study of rusted iron, seeks to corroborate the theory of putrefaction espoused by Kircher in his 1658 work. Hooke had published his landmark Micrographia; or, Some Physiological Descriptions of Minute Bodies Made by Magnifying Glasses in 1665. In this work, he attributes to the microscope a significant role in opening up the secrets of nature as it really is:
[We can add] artificial Organs to the natural, which has been of late years accomplisht with prodigious benefit to all sons of useful knowledge, by invention of Optical Glasses. By means of Telescopes there is nothing so far distant but may be represented to our view: and by the help of microscopes, there is nothing so small, as to escape our inquiry; hence there is a new visible world discovered to our understanding. . . . By this Earth it self, which lyes so neer us, under our feet, shews quite a new thing to us, and in every little particle of matter, we now behold almost as great a variety of creatures, as we were able to reckon up in the whole Universe it self.44
Enthusiasm for the microworld was shared by many of Hooke’s contemporaries; his book was received very positively and was much sought after and difficult to acquire. Leibniz himself, as Catherine Wilson notes, tried persistently for years to obtain a copy, receiving one finally only in 1678.45 Leibniz does, however, indicate in the 1669 letter to Thomasius, just cited, that he is familiar with some of its contents. “Putrefaction consists in little worms invisible to the naked eye,” he writes to his former teacher, adding that “any putrid infection is an alteration of man, a generation of worm. Hooke shows similarly in his Micrographia that iron rust is a minute forest which has sprung up; to rust is therefore an alteration of iron but a generation of little bushes.”46
At times, Leibniz seeks to diminish the importance of the microscope as a factor in the development of his ideas concerning the subvisible structure of physical reality, including his worlds-within-worlds doctrine. For instance, he claims at one point that the microscope did not discover for us anything we had not already known:
Although the conservation of the animal is favored by the microscopes, nonetheless we were aware of small bodies before their discovery, and thus we were already very well able to foresee the small animals, as Democritus foresaw the imperceptible stars in the Milky Way before the discovery of the telescope.47
In Leibniz’s animal-economical texts as well, treated in chapter 2, we do not find microscopy wielding any significant influence. In his work on medicine and animal economy, Leibniz seldom invokes the findings of microscopists such as Leeuwenhoek, Swammerdam, and others, with the same enthusiasm we see in his writings on what we have been calling “organics.” Why would Leibniz consider the apparent composition of animal bodies out of smaller organic bodies relevant to the broader philosophical questions treated in these texts, but not to medicine and animal economy? It appears that the model of the animal as nested is one that Leibniz perceived as being of little use to the practice of medicine, to which his interest in animal economy always remained subservient. With respect to animal economy, the microscope was just one of many tools that might assist in coming to know the workings of the body better, alongside purgatives, scalpels, and flesh-eating acid.
Leibniz mentions the usefulness of convex lenses in the De causis febrium of 1704–05, which in spite of its late date is remarkably consistent with and similar to the animal-economical texts of twenty years prior. Here Leibniz recommends inspecting a fluid “either with the naked eye or by means of a convex lens.”48 Microscopes are useful, but if the naked eye will work just as well, then it should be preferred. In spite of this preference in animal economy for unaided observation, the majority of texts in which Leibniz mentions the device suggest a belief on his part that it is an important tool in the search for knowledge of the natural world. He often notes that the microscope is useful for getting at the reality behind the appearances of colors: “We do not discern the blue and the yellow that enter into the representation . . . in the composition of green, though the microscope makes us see that that which appears green is composed of yellow and blue parts.”49 And he writes in the Discours touchant la méthode de la certitude et l’art d’inventer (Discourse concerning the Method of Certainty and the Art of Invention), written between 1688 and 1690, that “microscopes make us see, in the least atom, a new world of innumerable creatures, which help us to come to know the structure of bodies.”50 It is only a few years later that Leibniz will announce, in a letter to Huygens, that he is much more impressed with “a Leeuwenhoek” than “a Descartes,”51 that is, with someone who makes claims about how the world is after looking into its fine-grained structure rather than pronouncing that the world must be so and so on the basis of first principles.
Leibniz first looks through the lens of a microscope in 1676, thus some time after many of his core metaphysical principles have taken form. It has accordingly been assumed that, at most, microscopic discoveries might have served as empirical corroboration of doctrines such as that of nested individuality, at which Leibniz arrived, it is supposed, by means of strictly nonempirical reasoning. Prior to his visit to Leeuwenhoek in Holland in 1676, and before his acquisition of Hooke’s work in 1678, Leibniz’s acquaintance with microscopy was casual and secondhand. However, against Catherine Wilson’s claim that Leibniz only acquired knowledge of Hooke’s work in acquiring a copy of it in 1678, we have seen that Leibniz had already absorbed some of the content of the book by 1669 and, more importantly, that already in the late 1660s, as his metaphysical principles were still taking shape, Leibniz considered the evidence of microscopy—as presented, for instance, by the avid micros-copist Kircher—to be useful and informative in philosophy. Looking at (1) the citation from Hooke above, in which the microscopist describes the object of his study as “worlds within worlds,” six years before Leibniz’s first description of his antiatomist doctrine in these terms, and (2) at Leibniz’s 1669 letter to Thomasius, in which the student indicates to his former master familiarity with the microscopic research of both Hooke and Kircher, and (3) at the transformation in Leibniz’s later thought of the worlds-within-worlds doctrine into an explicitly microbiological thesis concerning the composition of macroorganisms out of worms or microorganisms, it seems plausible to suppose that the worlds-within-worlds doctrine, or the model of nested individuality, was inspired by microscopy from the very beginning.
Some authors have rejected as implausible the view that microscopy may have been part of the initial inspiration for Leibniz’s model of organic bodies for the simple reason that Leibniz, as a philosopher, had to have been drawing his inspiration from, as Leibniz himself puts it, “higher principles.” This conviction can at its worst lead commentators to muddle the historical facts about Leibniz’s philosophical development and his knowledge of empirical science. Hans Poser, for example, correctly discerns evidence of the doctrine of worlds within worlds as early as the De conditionibus of 1665, and, more clearly, in the De arte combinatoria of 1666, “thus long before the discovery of microorganisms by Leeuwenhoek in 1672.”52 From this, Poser concludes that when Leibniz finally looked through a microscope in 1676, “it was for him a matter of empirical corroboration of a principle which for him had to be true on primarily metaphysical grounds.”53 But Poser is simply mistaken in his claim that microorganisms were unknown in 1665; it was only a particular kind of microorganism, the spermatozoon, that was discovered in 1672, seven years after Leibniz first considered some version of the worlds-within-worlds doctrine. As Hooke’s Micrographia clearly reveals, the microworld was already at the center of scientific attention by the time of Leibniz’s first formulation of the doctrine in 1665. Indeed, as we have already seen, Monconys reports of the presence of “an infinity of minute invisible insects” in the human body as early as 1647.54 One of the people known to have influenced Leibniz very early on, Kircher, had already conducted decades of microscopic research and developed a theory of putrefaction involving invisible insects by 1658. In 1669, Leibniz expresses agreement with this theory, as well as indicating familiarity with Hooke’s work. To suggest that Leibniz could have had no idea of the existence of microorganisms earlier than 1672 is simply wrong.
We have the positive conclusion that Leibniz’s mature period is characterized by the appearance of explicitly biological terminology to describe the worlds-within-worlds doctrine as well as some evidence that Leibniz was interested in microscopy during his early, formative period. The explicit biological character of Leibniz’s later thought, the emergence of this later thought directly out of the earlier thought, and the evidence of interest in biological nestedness and in microscopic creatures in the young Leibniz’s writings together suffice as grounds to take seriously the hypothesis that microscopic discovery played an important role in the development of Leibniz’s metaphysics of corporeal substance. Naturally, there were numerous factors influencing Leibniz’s model of organic bodies, many of which had nothing to do with any of the empirical sciences. But there is evidence that microscopy was among these factors.
This evidence grows, moreover, as our knowledge of Leibniz’s medical and natural-scientific manuscripts of the Paris period and earlier grows. One such manuscript, likely from the end of Leibniz’s stay in Paris, gives a vivid account of his awareness of the nestedness of invisible creatures within visible ones:
Tschirnhaus believed himself to be able to show that no animals are not from animals that do not arise from other animals. Fleas and lice of various sorts are produced in various animals; various animals are able to produce themselves, where no animals were before. Redi performed experiments, [and] if he had followed them through, he would have arrived at this. There are at length certain black spots in men’s nostrils, and if they are pressed a sort of worm can be squeezed out of them, a larva, and this is confirmed by the example of pediculosis,55 but these perhaps demonstrate as much. Visible animals often arise from invisible ones that make the air full, from undetectable spiders [come] these webs that fly through the air, which are called vulgarly Mariengarn [“Mary’s thread”].56
In all probability, this text was jotted down prior to Leibniz’s visit to Leeuwenhoek in Delft in 1676. But the sort of discoveries that the microscopists were making were already, like so many Mary’s threads, floating in the air, and it is not at all surprising that Leibniz took notice of them.
Leibniz notices something close to parthenogenesis, or reproduction by budding, in a curious letter to Arnauld of 1687: “I dare not maintain that plants have no souls, nor life, nor any substantial form: since, although one part of a tree planted or grafted can produce a tree of the same kind, it is possible that there is in it a seminal part which already contains a new plant.”57 Yet he strictly denies that animals can reproduce by splitting in the same way that plants can on the grounds that animals have a complicated structure, such that not every part may be said to contain the principles required for the formation of the whole: “A twig of the plant is often capable of producing a new and complete plant, for which we see no analogy in the animals: thus we cannot say that the foot of an animal is an animal, in the way that it seems that each branch of a tree is a plant capable of flourishing on its own.”58
Unbeknownst to Leibniz, however, Leeuwenhoek would fully accept the reproduction of animals through splitting. The Dutch microscropist describes this process in minute detail in a letter of December 1702, recounting his experiments of several decades earlier:
[I saw] an animalcule coming out of the first animalcule, and when heretofore I saw such an animalcule attached to a larger one, I thought that a younger animalcule was attached accidentally to a larger one, but on closer observation I saw that it was a generation, for I observed that, while the last-mentioned animalcule, when first I saw it was an animalcule, had only four very small, short tentacles, after sixteen hours the animalcule had grown as to the size of its body and tentacles, and after four hours more I saw that it had left its mother’s body.59
As we have seen above, in response to a question from Arnauld concerning the division of insects, Leibniz seems skeptical of the suggestion that both parts remain truly living. While Leibniz here passes up the opportunity to turn the question of parthenogenesis into one of philosophical significance, this passage is certainly interesting in that it indicates the widespread observation of division in the animal world prior to the eighteenth century, favoring the view that there was a long heritage of parthenogenetic experimentation, against the widespread argument that parthenogenesis went more or less unacknowledged prior to the eighteenth century.60 It is at least true that parthenogenesis did not become the source of widespread philosophical speculation until the 1740s. It was Abraham Trembley, with his announcement of the discovery of reproduction by division in polyps, in May 1741, who brought parthenogenesis to the center of the scientific community’s attention.61 Having chopped a polyp into several small pieces, grafted bits of polyp onto other bits, and even turned what looked to be a whole polyp inside out, so that what was thought to be the creature’s stomach quickly became its epidermis, Trembley reasonably concluded that there could be no sense in speaking of polyps as fixed, enduring, organized individuals.62
A short time after the freshwater polyp had become a common touch-stone of natural science, the French naturalist and germ theorist Charles Bonnet wrote in his Considérations sur les corps organisés of 1762 (unaware, of course, of what Leibniz had written in the correspondence with Arnauld) that “the metaphysics of this great man [Leibniz] led him to suspect the existence of such a being as the polyp.”63 Bonnet would hypothesize that “the polyp’s body is, so to speak, constituted by the repetition of an infinity of small Polyps, who are only waiting for favorable circumstances in order to come to the light of day.”64
By the time Bonnet wrote this, the biologist and natural philosopher Pierre-Louis Moreau de Maupertuis had already appropriated the term “monad” and turned it into a physical atom endowed with a “per- ception élémentaire.”65 Within a few more years, in 1773, Müller would give the genus name Monas to a number of newly discovered species of infusorium.66 And throughout the first half of the nineteenth century, C. G. Ehrenberg and other microbiologists introduced to the world a number of new Monas genera and species, including the Cryptomonas or Panzermonade, and the elusive Monas minima.67 Ehrenberg revealingly describes members of the genus as “point-animals [Punktthierchen],”68 while Müller defines “monad” in his work as a “Corpus punctiforme.”69 And of course no survey of the eighteenth-century physicalization of the monad would be complete without a mention of Kant’s 1756 work, Monadologia physica. This title is striking, but as we are seeing here Kant was by no means alone in conceiving monads in this way. In the decades following Leibniz’s death, monads were broadly reconceived after the model of the infinity of small polyps that made parthenogenesis possible. They had become living and physical building blocks of the natural world.
According to the canonical view of Leibniz’s philosophy today, all these eighteenth-century thinkers would seem to have missed the point: the monad was meant to be an atom of substance, a metaphysical simple endowed with nothing but perception, and certainly not a tiny speck of living matter or a punctiform body. It was with this in mind that Jacques Roger wrote that “in general, the eighteenth century,”—by which Roger meant principally eighteenth-century natural scientists—“understood Leibniz’s metaphysics poorly.”70 To be sure, physical monadologists entirely abandoned the distinction that Leibniz considered so crucial between “atoms of substance” on the one hand, from which bodies result but not from which bodies are built, and physical atoms on the other. Yet Bonnet does seem right to say that parthenogenesis may be seen as a sort of confirmation of Leibniz’s model of organic body. For what else is the small polyp awaiting “favorable circumstances in order to come to the light of day,” as Trembley writes, but a nested individual that, as a result of its currently being dominated by the principle of activity of the polyp of which it is an element, has yet to be fully activated?
The tremendous difference between the parthenogenetic model of organic body and Leibniz’s model is that the polyp, as conceived in the eighteenth century, is not in any sense a unified corporeal substance. Leibniz can thus be seen as representing a halfway point between a certain widespread premodern conception of biological entities, which sought to give an account of the entity’s unity or soul-based properties while by and large ignoring its microphysical make-up, and the modern conception, which would come to see the microphysical make-up as all there is to an animal, as the only feature of a living being that differentiates it from nonliving entities, while its unity would come to be seen as nothing more than a temporary, contingent supervenience upon the physical make-up. In this sense, we may say that Leibniz’s theory, focusing as it does on unity and on microstructure, straddles the boundary between the old metaphysical picture of animals and the modern biological picture that emerged out of it.
By the mid-eighteenth century, a strict materialism would come to dominate, particularly in France but also to no small extent elsewhere in Europe, in the philosophical discussion of what a living being is. On this view, espoused in its most radical form by Julien Offray de La Mettrie in his L’homme-machine (1747), Claude Adrien Helvétius (the grandson of the discoverer of ipecac, discussed in chapter 1) in his De l’esprit (1758), Paul Henri d’Holbach in his Système de la nature (1770), or Denis Diderot in Eléments de physiologie (1784), animals, including humans, would come to be seen as mere clumpings of matter that, for a limited time, through a fortuitous arrangement of their parts, come to develop the capacity for sensation and motion, and, in some few cases, for self-awareness. On this materialist picture, there is no room for an ontological distinction between real, existent entities and phenomenal semi-entities. Leibniz’s insistence on a soul-based principle of unity for the explanation of all organized, living matter, rather than taking spirit itself as a supervenient property of organized matter, would come to be viewed by materialists such as La Mettrie as unjustifiable speculation.71 The living being would come for many in the eighteenth century to have no more reality than the reality of its parts. In other words, in Leibniz’s terms, living beings would come to be seen in the eighteenth century as aggregates, or, in Robert Sleigh’s terms, as divisible rather than just component-wise deconstructible.
Diderot explains in his Eléments de physiologie of 1784 that life consists in an arrangement of organs. Insofar as the organs can be separated from one another and, in some cases, continue living, whatever there is in the animal that might be called a “soul” is not indivisible. He asks, if “life remains in the organs separated from the body; where then is the soul? What becomes of its unity, its indivisibility?”72 Insofar as ensouledness is for Diderot just the temporary capacity for motion and sensation, any division of an animal body that results in the deprivation of this capacity to one or both of the parts of the body constitutes an empirical demonstration of the possibility of the separation of the body from the soul. “A ligature of the nerves impedes all sensation, all movement,” he writes, “a ligature can thus separate the soul from the body.”73
In his 1784 work Diderot goes so far as to describe all animal reproduction as consisting in the division of one organ from another, as the passing on of life or ensouledness from one bit of living matter to another. This communication of life does not result in the coming into being of a new individual. For Diderot, generation consists in the rearrangement of matter, and, eventually, in the physical separation of this matter from the more massive source matter. He explains that “the generation of parts occurs little by little, and not suddenly, through the arrangement of parts, and not by development.”74 While in an earlier treatise Diderot had joked about humans reproducing by parthenogenesis, he later suggests that sexual generation may be nothing more than the separation of a quantity of living matter from another quantity. He writes that he is “tempted to assimilate the generation of man to that of the polyp that reproduces by means of division. The union of the man and the woman only gives rise to the production or the development of a new organ.”75
For Leibniz, as we have seen, even if both halves of a bisected corporeal substance go on living, the corporeal substance itself has not been divided. It survives in only one of the halves, while the other half falls under the domination of a new, previously subordinate substantial form. Without germs or dominant monads, there is no question as to which half of a bisected worm or polyp (or human) remains the same creature that the whole had been prior to the bisection. On Diderot’s interpretation of parthenogenesis, the whole prior to division was not really a whole at all, but rather, in Leibnizian terms, an aggregate. On this view, the whole animal functions as one, but has no more real unity than a team whose members may quit at any time. When a member quits, for example, or when a fetus separates from its mother, or when a bit of a worm is cut off from the rest of the worm, there is no question for Diderot as to which bit of living matter retains the soul, for soul is for him nothing more than the capacity of matter for sensation and motion. The presence of soul in living matter does not elevate this matter to the status of substance, as it does in Leibniz’s corporeal-substance metaphysics. There is for Diderot no basis in an incorporeal principle for determining to what substance some bit of living matter belongs. Thus, for him, there is no better answer to the question, “which part of the bisected worm is the worm that was here prior to the bisection?” than to the question “which chunk of the cleaved block of marble is the block of marble that was here prior to its cleavage?”
In the eighteenth century, then, Leibniz’s monads become assimilated to microorganisms. But were microorganisms involved in his original invention of them? In the same measured tone earlier employed by Jacques Roger, Catherine Wilson has suggested that “we need to see the monads as to some extent modelled after the animalcula—they have perception and appetition, and can exist at very low degrees of awareness and general competence.”76 Wilson thinks that there are important differences between the middle-period metaphysics, in which Leibniz thought of all objects as aggregates of animalcules,77 and the later metaphysics, in which Leibniz thought of objects encountered “in” space as “the representations of perceivers, each of whom experiences its own “world” as a more or less adequate version of the world perceived by God.”78 Yet, she notes, throughout the most mature, immaterialist period, Leibniz repeatedly falls back on a stock image of the world as akin to “a garden full of plants” or a “pond full of fish”:
Leibniz did not want to give that fish-pond, the most arresting image of the whole Monadology, up, and who can blame him? That it was apparently irreconcilable with his immaterial atomism troubles his modern commentators; Leibniz himself seemed to combine a confidence that his two systems really did converge with an anxiety about the details required to work this convergence out.79
One may agree with Wilson while nonetheless clarifying that the monads themselves, for Leibniz, are by no means basic building blocks in the way that cells are today held to be the building blocks of organisms. Leibniz’s animalcula or worms were not conceived on the model of cells but rather as organic bodies with no lower limit to their composition, like a worm that might be split into infinitely many worms. Rather than holding that the monads can be seen to some extent as modeled after the animalcula, we might do better to say that it is the organic bodies of corporeal substances that are modeled after animalcula. Leibniz’s central concern in developing his model of organic bodies, is to argue that any appearance of discreteness or self-containedness in a biological entity is only an appearance. The monad is unlike the animalcule to the extent that it is a truly simple and self-contained unity. The animalcule, in turn, is like the organic body in that neither is at all self-contained but both are always composed out of other organic bodies. As we have seen, the model of “animalcule” from which Leibniz draws inspiration is decidedly not that of a basic-level, cell-like organism. It is a “worm,” which Leibniz seems to conceive, at least from the Arnauld correspondence on, as capable of surviving bisection, which is to say, in the terms of Leibnizian metaphysics, as capable of allowing this or that previously subordinate monad to rise to predominance as a result of changes in the structure of the organic body.
What is characteristic of the corporeal-substance metaphysics is not that Leibniz thinks of complex entities as being constituted out of a number of basic, cell-like, not-further-decomposable entities. Rather, Leibniz believes, along with many of the microscopists, that there is nothing fundamental or bottom-level about microorganisms at all. The lesson that enhanced perception taught was, first and foremost, a humbling one: one must never presume that one has arrived at the bottom level. An animal-cule, like a horse or a man, is composed out of other animalcula, which are themselves composed out of other animalcula, and so on without end. When monads are introduced, in contrast, this is in large part so that there may be a lower limit to the analysis of things, even if this limit is immaterial rather than atomic in the traditional sense. The animalcula, then, are never basic, whereas the monads are so by definition.
The picture of bodily matter as lacking any basic building blocks is buttressed, rather than contradicted, by the subvisible world of animal-cules described by the microscopists. Certainly, it is easy for us, today, to think that the opposite picture of matter would be inspired by such a discovery, in view of our common knowledge that there is a basic unit that constitutes living beings, namely, the cell. Yet it would be another century and a half before the cell would be isolated as the basic unit of the living being.80 In the mid-seventeenth century, the astonishing implication of the microscope was that living nature seemed to lack a bottom limit in its composition. What had once been thought of as small and simple, the worms and insects visible to the naked eye, turned out to be, on closer inspection, fully outfitted with complex organs that function just as ours do. Any lower limit one might impose is only a result of the contingent fact that our optical organs are capable of detecting things only within a certain range of sizes. If the range were different, our conception of what is small, and our estimation of roughly how small the smallest living entities are, would also be different.
Why, then, is it hard to break free of the anachronistic idea that with his monads Leibniz had something like animalcula in mind, and with his animalcula, something like cells? Here, as perhaps elsewhere, we remain under the spell of earlier generations’ agenda-laden readings of Leibniz. As Georges Canguilhem has observed, and indeed as we have already seen, by the end of the eighteenth century, “the term monad was frequently employed to designate the supposed [basic] element of the organism.”81 Leibniz could not have had anything like the cell in mind when he introduced the concept of monad at the beginning of his mature-period metaphysics any more than when he took an initial interest in microscopy, around 1669. It is true that Hooke speaks of “cells” in his influential Micrographia, but what he has in mind are empty spaces or pockets in certain materials he examined; thus for Hooke a “cell” is literally akin to the monk’s small living space in a monastery from which it derives its name. When Leibniz appears to shift from talk of animalcules to talk of monads, this is not, as Wilson has described it, “a continuation of policy by other means.”82 Rather, the animalcule is never, for Leibniz, a basic building-block entity; it is always further divided into subordinate constituents. The monad, in contrast, is always a basic building-block entity, even if the bodies of corporeal substances are not literally built from them but rather result from them. Animalcules are infinitely divided and lack, qua organic bodies, any rock-bottom principles of their own; monads are simple and immaterial and yet are themselves the rock-bottom principles from which organic, animalcular bodies, indeed all bodies, result.
Leibniz, as we have seen, continues throughout his late period to speak of worms and other such animals, and these are never in competition with the monads that are supposed to come and replace them and to partially continue their middle-period work “by other means.” Rather, these worms are the organic bodies of corporeal substances or “animals,” and they are infinitely divisible (or, more precisely, deconstructible in Sleigh’s sense) rather than being basic building blocks. Considered apart from the corporeal substance with which they are united, they are an infinite aggregate of smaller organic bodies; considered as a corporeal substance, they are an infinite union of dominated and dominating corporeal substances. In the end, corporeal substances are but the result of the primitive active and passive forces of monads, but it is for the infinitely deconstructible organic bodies of these corporeal substances that the animalcula or worms may be said to serve as an inspiration, and not for the monads that in the end underlie these bodies. Leibniz writes to Malebranche as early as 1679:
There is even room to fear that there are no elements at all, everything being effectively divided to infinity in organic bodies. For if these microscopic animals are in turn composed of animals or plants or other heterogeneous bodies, and so on to infinity, it is apparent, that there would not be any elements.83
This letter is a harbinger of things to come, and indeed an early outlier of a line of explanation that fifteen or so years later Leibniz would begin to repeat with increasing frequency and with increasing explicitness as to the biological nature of the beings implicated in the constitution of larger organic bodies.
To return to the question with which we began this chapter, then, we may now be in a better position to answer the question: why the infinite nestedness? We may agree with Roger that Leibniz’s theory of organic body is motivated by a desire to take account of the real. But we still must ask, why does “the real” come out looking precisely like that? The answer seems to have much to do with the state of microscopical and anatomical research in the scientific context out of which Leibniz’s philosophy emerged.