In chapter 3 we saw how Leibniz manages to account for the growth, development, and motion of animals by appealing to “material plastic natures” alone. Nevertheless, the question of the original formation of plants and animals, and other animal-like corporeal substances, remains. Ralph Cudworth had resorted to the doctrine of immaterial plastic natures because he had assumed that organic bodies did not exist prior to embryogenesis. Something therefore had to be introduced initially in order to bring living beings into existence. In this, Leibniz likens Cudworth’s doctrine to that of those who believe that it is the soul that is responsible for making its own body: “I have no need to resort, as does Cudworth, to certain immaterial plastic natures, though I recall that Julius Scaliger and other Peripatetics, as well as certain adherents of van Helmont’s doctrine of the archaeus believed that the soul makes its own body.”1 The soul does not make the body, for Leibniz, because the body is always already there. But how could Leibniz have believed such an improbable thing? And to what extent is this belief rooted in his interest in the science of animal generation?
It is clear that as early as the period of his major animal-economical texts, considered in chapter 2, Leibniz had developed an intense interest in the technical and empirical science of generation, quite apart from his emerging a priori commitments to the ungenerability of corporeal substances. Thus he writes to Gunther Schelhammer in December 1680, on the difficult problem of the causal relationship between intercourse and conception:
Concerning the . . . seminal matter, it is to be investigated whether this is equally necessary to the generation of animals. Certainly, that the [seminal] matter in the testes is generally necessary is apparent from the example of eunuchs. Of the matter that comes from the prostate there is room for doubt. And truly the problem of highest importance of all in this argument is to discover why at times conception should occur, and at other times coitus should turn out to be in vain. That is, what are the true requisites of conception?2
In this correspondence Leibniz is interested not just in human conception but in reproduction across the animal kingdom. Thus in the letter just cited he goes on to ask “whether dogs that have been deprived of their spleen are sterile,” and in a letter from September of the same year he notes that he has read “with much delight” those things that Schelhammer relates “concerning the genitals of moles [talpae].”3 “Lower” animals with newly discovered genitals were important for empirical refutation of the view that such kinds generate only spontaneously, while eunuchs were important as a comparison class with “normal” men in the effort to understand what the requisites of conception are. But these empirical questions would give rise to other, more properly philosophical ones. For example, what is it in the fluids in ordinary cases of sexual reproduction that is imbued with the power of generation? What, moreover is the nature of this power? Does the fluid conceal some primordium that would reveal that what we call generation is not generation at all, but only, as Leibniz would put it, an entry into “the larger theater”?4 In this chapter we will consider Leibniz’s contribution to generation theory in its philosophical context as a central problem of natural philosophy as well as in the context of seventeenth-century empirical research.5
In much of the popular literature on childbirth and midwifery from the late Middle Ages into the seventeenth century, a caricatured version of Aristotle’s view of sexual reproduction as an “approximation of eternity” had become a commonplace. Apocryphal treatises “de secretis mulierum” and their vulgate translations proliferated throughout the thirteenth and fourteenth centuries and were attributed variously to Aristotle or to his medieval commentator, the philosopher and zoologist Albertus Magnus.6 One late-coming contribution to this genre was that of François Mauriceau, who in the years between Descartes’ mechanical epigenesis and Leibniz’s preformation theory, wrote for a broad audience of surgeons, midwives, and other curious readers:
It is a very great truth, and one recognized by all of us, that everything that is in this low world is subject to corruption, and in the end is constrained to suffer death. This is what obliged nature, provident and concerned for its own conservation, to give to each creature a certain desire to make itself eternal, which, not being able to be done in the individual, insofar as he is mortal, is done by an indispensable necessity through the propagation of forms and species.7
Although directed at a popular audience, Mauriceau’s account of sexual reproduction well epitomizes the predominant premodern account of embryogenesis, according to which this process is governed by an immaterial principle of development that realizes form in matter to the extent possible or appropriate for a given kind of creature. This is a view that would continue to be defended explicitly by sophisticated theorists well into the seventeenth century, and that, as we will see, would continue to implicitly guide the accounts of embryogenesis of theorists who directed much of their energy toward denying the need for such an immaterial principle.
One seventeenth-century theorist who continued to explicitly defend such an account was William Harvey, the English Aristotelian and former student of the Padua school of medicine. For him, as he explains in the De conceptione, published as an appendix to the 1651 Exercitationes de generatione animalium,8 the homonymy between biological and cognitive “conception” is more than just homonymy; he argues that what occurs in the uterus at conception is literally the same thing that happens when an idea is had in the brain.9 Once the idea is formed in utero, it is then fashioned with the available material—the menstrual blood—into a creature resembling its progenitors. While the environment may play a role in the acquisition of particular traits, it does not have to, on Harvey’s understanding, in order for the conceived offspring to come out more or less like its parents, since its development is guided ab initio by an idea of its parents’ traits. Insofar as Harvey is able to rely on an internal, immaterial principle guiding the fetus toward a particular end, namely, that of being a particular sort of creature with particular traits, he is under no pressure to postulate external, efficient causes to account for species reproduction and trait acquisition. Harvey maintains that there is nothing corporeal left in the uterus after coition, and infers from this that fertilization must consist in the activation of an incorporeal principle of development. Thus, while justifying his position in terms of empirical evidence, Harvey remains faithful to a premodern conception of sexual generation.
Accounts of fetal development as an end- or idea-driven process were doomed to fall into disfavor in the seventeenth century. However, it would prove much harder to eradicate immaterial, guiding principles of development from embryological explanation than many modern thinkers would have liked. This is because, as the example of Harvey makes very clear, such principles would continue to appear indispensable in the explanation of organic phenomena long after physics, the study of inorganic bodies, had proved, for many, perfectly able to do without them. Leibniz, rather than deviating from his mechanist convictions and reintroducing plastic natures or some other immaterial principle of development, takes a different tack: for him, as for Malebranche, there can be no mechanical explanation of the coming-into-being of animals, yet both philosophers also believe that animal bodies can be explained in terms of mechanical causes to no less an extent than other entities in nature. There is no contradiction between the universality of mechanism, on the one hand, and the nonmechanicity of generation on the other, in Leibniz’s view, insofar as animals are never generated at all, but have preexisted since the creation:
Animals never having been formed naturally from an inorganic mass, mechanism is incapable of producing these infinitely varied organs anew, though it can take them by means of development and transformation from a preexisting organic body.10
Indeed, Leibniz is so committed to mechanism that he takes the evident impossibility of a mechanistic explanation of the generation of animals not as a refutation of mechanism, but, on the contrary, as strong evidence for the view that generation must not actually occur. It is only insofar as corporeal substances are never formed at all that Leibniz is able to agree with Cudworth that corporeal substances such as animals cannot be formed mechanically while maintaining nonetheless that whatever happens, happens mechanically:
Plants and animals . . . are never formed entirely anew. I am thus of the opinion of Mister Cudworth (whose excellent work for the most part pleases me very much) that the laws of mechanism alone would not be able to form an animal where there was nothing already organized; and I find that he is correct in opposing that which some of the ancients have imagined in this matter, and even what Monsieur des Cartes has imagined with his man, the generation of whom costs so little, but who resembles so little a true man.
In other words, what Descartes’ embryology succeeds in accounting for is at best a mere machine, not a divine or natural machine, let alone an ensouled corporeal substance. Leibniz continues:
And I strengthen this opinion of Mr. Cudworth in that I consider that matter, arranged by a divine wisdom, must be essentially organized throughout and thus that there are machines in the least parts of the machine to infinity.11
Leibniz’s careful choice of words reveals the deep interconnection of his theory of organic body and his commitment to preformation: as against Descartes, mechanical laws alone could never account for the origins of a new natural being; yet, as against Cudworth, Leibniz wishes for his account to remain broadly compatible with traditional mechanism, so rather than invoking a force or principle in matter capable of bringing new organization into being where before it did not exist, instead Leibniz argues that it is simply of the nature of organic body to have no origin in time (other than at the Creation).
In his Considerations on Vital Principles and Plastic Natures, Leibniz remarks that he neither needs Cudworth’s immaterial plastic natures, nor do they fulfill his needs since, as we have already seen, “this preformation and this infinitely complex organism” will do the job just as well. The material plastic nature consists for Leibniz in the organic body alone, which is infinitely complex. Because it is infinitely complex, no process of decomposition may ever take it out of existence, nor bring it into existence. It follows from this, Leibniz believes, that the organically embodied creature must always have existed, unless it was created miraculously directly by God, an intervention Leibniz refuses to invoke for the explanation of nature’s ordinary operation. Since no organically embodied creature ever comes into existence, we do not need to appeal to plastic natures to account for coming-into-existence. He explains:
I believe that not merely the soul but the whole animal subsists. Very exact observers have noted before now that it is doubtful that an entirely new animal is ever produced but that living animals as well as plants already exist in miniature in the seeds before conception. Assuming this doctrine to be true, we may reasonably conclude that what does not begin to live does not stop living either and that death, like generation, is only the transformation of the same animal, which is sometimes augmented and sometimes diminished. Thus, we discover the marvels of divine artifice even where they have never been thought of. For since the mechanisms of nature are mechanisms down to their smallest parts, they are indestructible, since smaller machines are enfolded in greater machines to infinity. Thus one finds oneself forced to maintain, at the same time, both the pre-existence of the soul with that of the animal and also the subsistence of the animal with that of the soul.12
While what Leibniz is emphasizing is the preexistence of the whole substantial animal, it is a necessary condition of such preexistence that there be an accompanying preformed organic body that is by definition infinitely complex, as we already saw in chapter 3, and therefore undecomposable. No sequence of cuts can bring it out of existence, and by the same token no sequence of juxtaposition of parts can bring it into existence. Thus the whole animal is not only component-wise deconstructible and substance-wise indivisible; it is also—to introduce two more cumbersome neologisms—component-wise supplementable and substance-wise ungenerable. Leibniz frequently emphasizes that it would go beyond the limits of the possibilities of mechanism to propose that an organic body could ever come into being where there was not one before: “The mechanism, incapable of producing de novo these infinitely diverse organs, can very well attain them from the development and transformation of a preexisting organic body.”13 In sum, Leibniz saves his own mechanism by opting for a preformationist account of generation.
Before proceeding any further it will be useful to make a terminological clarification, one that sets the present study at odds with recent scholarship. Preformation theory, as it is understood here, is the view that development consists merely in the expansion or unfolding of structures that exist prior to conception. Epigenesis, in contrast, holds that the development of the fetus proceeds through increasingly complex structures. Epigenesis has triumphed in modern developmental biology since the zygote is undifferentiated, and different types of cells, those of nerves, muscles, and such, develop only gradually as a result of conception. The debate between the epigenesists and preformationists became the central problem of embryology in the early modern period, at the moment when preformation gained new plausibility as a result of microscopic research on the primordia of living beings.
Most scholars, following the groundbreaking studies by Jacques Roger and Peter Bowler,14 followed by the influential work of Andrew Pyle,15 make a further conceptual distinction between preformation and preexistence theory. Only the latter view for these authors has it that all creatures must exist since the creation, and that generation is only the unfolding of what has always already existed. Preformation, in contrast, for them holds that the fetus is formed before conception at some particular time, but not necessarily by God, and perhaps even by some natural means.
Leibniz’s theory involves not just some preexisting entity or other, but an entity with a particular organic form—at least in the mechanized sense of this notion in which Leibniz often understands it, as synonymous with “figure” or “structure”—and to speak of preformation rather than preexistence brings out more accurately this feature of Leibniz’s theory. Preexistence, in contrast, could in principle describe a belief in disembodied souls in some “baby heaven” awaiting their descent into a bodily life at conception, a possibility that Leibniz is very concerned to rule out. François Duchesneau has rightly emphasized that what exactly it is that is preformed is a difficult question to answer, since, as we will see later in this chapter, Leibniz repeatedly emphasizes that a tremendous change, or metamorphosis, is required to bring the creature from its preexisting state into the larger theater. But there is one thing about which Leibniz could not be any clearer: there is no coming-into-being of creatures, and every creature always exists in an organically embodied form, supernaturally brought into being simultaneously with the rest of creation. The term that Leibniz uses to describe this view is “preformation” (or, more precisely, “divine preformation”), and not either preformationism or preexistence. We shall follow Leibniz’s usage here, in full recognition that it deviates from the distinction later made by Roger and Bowler.
Toward the end of the seventeenth century, preformation in one version or another enjoyed much greater currency than the opposing view, only to decline rather swiftly in the eighteenth century in the wake of the debates between Caspar Friedrich Wolff and Albrecht von Haller.16 The popularity of the preformationist view in this period, whether “ovist” or “animalculist,” was motivated not only by the evident limits of mechanical epigenesis but also by the century’s discovery of subvisible organisms that might appear as promising candidates for the role of primordia.
Leibniz’s preformation theory provides a model of animal bodies as machines, but these are a very peculiar sort of machines, to the extent that there is no possibility of manufacturing them. This is a distinction that makes an ontological difference. Descartes seems to have well understood as much and to have seen the success of his theory of the animal-machine as hanging upon his ability to successfully account for the origins of animals “in the same style as the rest, namely, by demonstrating effects from causes.”17 He could not provide such an account, and for precisely this reason, in retrospect, Descartes’ embryology appears wildly speculative and ungrounded. Yet Descartes had believed that his own epigenetic account of embryogenesis through thermomechanical causes must be true for deep philosophical reasons, even if its details remained to be worked out.
The failures of early mechanists such as Descartes to provide such an account were often derided, and conception and embryogenesis were often explicitly cited, by later seventeenth-century philosophers who wished to reintroduce plastic natures, archaeus, formative virtues, and such, to account for the phenomena of conception and development. An amusing example of such a view of mechanist generation theory comes from John Ray, who writes in his Wisdom of God Manifested in the Works of His Creation of 1691, that generation
is so admirable and unaccountable, that neither the Atheists nor Mechanick Philosophers have attempted to declare the manner and process of it; but have (as I noted before) very cautiously and prudently broke off their Systems of Natural Philosophy here, and left this Point untoucht; and those Accounts which some of them have attempted to give of the Formation of a few of the Parts, are so excessively absurd and ridiculous, that they need no other Confutation than ha, ha, he.18
Leibniz’s doctrine of preformation might perhaps be seen as just such a prudent breaking off, though denial that true generation occurs is something very different from simple neglect of it. To replace generation with development and unfolding is, in a certain sense, to address “the manner and process of it.” Nor could Leibniz be accused, though he remains a “Mechanick Philosopher,” of offering an excessively speculative account. For, though he often claims that his account of generation is derived from a priori principles, Leibniz never hesitates to draw upon empirical evidence that he takes to corroborate his views.
Let us recall Leibniz’s declaration, previously cited, of his preference for “a Leeuwenhoek” who tells him what he sees, over “a Cartesian” who tells him what he thinks. We might approach Leibniz’s engagement with generation theory and its place within his philosophy as a whole by asking whether he comes to hold the theory he does as a result of “seeing” or “thinking.” That is, we might ask whether Leibniz’s generation theory is relatively more Leeuwenhoekian than Cartesian, or the reverse.
Views have changed over the course of Leibniz’s reception history as to the relationship between empiricism and a priori commitments in Leibniz’s generation theory.19 In a curious polemical work dating from 1741, Eine bescheidene Prüfung der Meinung von der Præexistentz oder dem Vorherseyn menschlicher Seelen in organischen Leibern (A Conclusive Examination of the Opinion concerning the Preexistence of Human Souls in Organic Bodies), Johann Friedrich Bertram insists “that the proposed preexistence of human souls in organic bodies is nothing but a whimsically adopted poem.”20 He wonders:
Why were so many millions of souls created so incredibly long ago by the Creator, without any purpose. . . . Were they all ready to be brought forth at the beginning of the world? . . . Did there not occur in the meantime a great drifting of the souls? Did not some wind blow them to and fro, some foot press them into the mud, some mouth grind them with its teeth, and was not a great, uncountable mass of human and cattle bodies consumed with food and drink? . . . Not to mention other enormous absurdities.21
Indeed, put in these terms preformation does seem rather absurd. Who could have been responsible for the promulgation of such a fairy tale in the modern era? Bertram thinks he knows: “The person who helped this opinion onto its feet . . . is the famous Herr von Leibniz, a man who is known well enough both for his broad erudition as well as for all sorts of peculiar and extravagant ideas.”22 Most peculiar and extravagant from Bertram’s point of view is not the view that souls preexist, but rather that they exist in organic bodies; for him, Leibniz’s theory of divine preformation is most offensive because it denies “the immaterial nature of our souls,” and holds instead that the soul “is connected with a small, eel-shaped body [Ahl-förmigen Cörper].”23
Bertram is somewhat forgiving, identifying the source of Leibniz’s absurd belief not in Leibniz’s imagination alone but in the imagination of one of Leibniz’s contemporaries:
But (as we recall in the aim of excusing him) Herr von Leibniz would never have come to the fantastic concepts of his monads and little spirits, if the famous Leeuwenhoek in Holland hadn’t discovered such strange things with the microscopes he invented.24
From this polemical work we see again that by looking to Leibniz’s immediate successors, a picture of Leibniz emerges on which his philosophical views are forged out of direct engagement with empirical life science. It is certainly an exaggeration to say that Leibniz never would have come to his preformationism had he not encountered Leeuwenhoek. Yet the particular form that Leibniz’s theory of divine preformation took is one that likely would not have been possible prior to the emergence of microscopic science.
As has been emphasized already, the relative successes of the emergence of mechanical natural philosophy look very different depending on whether one is considering the physical sciences or, instead, what would later come to be grouped together as the life sciences. Perhaps the most problematic subdomain of all the life sciences is generation theory, or the effort to account for the coming into being of new living beings. Indeed, with respect to the problems of generation theory, in an important sense the Aristotelians possessed much more powerful conceptual tools. As Mauriceau’s late echo of their account attests, sexual reproduction fit perfectly into a broader cosmological scheme in which form and matter play their respective roles, and in which male and female reproductive principles are but instances of these all-encompassing roles. However, notwithstanding the empirical work of Galen (on the reproductive tracts of Barbary apes, rather than humans), for the most part ancient generation theory was based on “thinking” rather than “seeing.” Indeed, until Harvey performed his methodical autopsies of deer in various stages of pregnancy, no one had directly, systematically observed the process of mammalian embryogenesis. For the first time in the latter half of the seventeenth century, then, it became necessary to make the thinking and the seeing that constituted generation theory match up. It is clear that as a result of the new possibility of coordinating thinking and seeing, many had begun to feel that new empirical discoveries had advanced generation theory beyond its earlier guesswork. Thus George Garden writes in the Philosophical Transactions of January 1691, on the subject of the generation of animals that “Mens Conjectures were . . . wide and unsatisfying upon this Head until this Age.”25
Leibniz would certainly reject Bertram’s claim that not only his theory of preformed organic bodies but also his theory of monads owes its origins to the discoveries of Leeuwenhoek. Leibniz explicitly claims on more than one occasion that his commitment to the doctrine of preformation comes not from any particular microscopical discovery, but from a decidedly philosophical source. As he explains to Burchard De Volder in a letter of June 20, 1703: “No entelechy ever lacks an organic body. As far as my consideration of these matters goes, things could not be otherwise; they are not derived from our ignorance of the formation of fetuses, but from higher principles.”26 He will later claim in the Monadology that his a posteriori reasonings about the eternal existence of organic bodies, which are “derived from experience,” nonetheless “agree perfectly with my principles deduced a priori.”27 In other words, Leibniz is fully confident, or at least claims to be fully confident, that he would have come to believe in the doctrine of preformation no matter what the state of empirical science in his lifetime. Indeed, in an intriguing undated fragment published in the Otium hanoveranum, Leibniz criticizes Harvey for assuming that conception must result from a “spiritual radiation” simply because he was unable experimentally to determine the locus of it. Harvey cannot discover where conception occurs, Leibniz thinks, “because this [the process of conception itself] is disturbed by dissection. Whence, having discovered nothing, he imagined to himself that it occurs through spiritual radiation.”28 For Leibniz, the limitations on human intervention in the process of generation must not in themselves dictate what is to be deemed the superior theory of generation. Far from acknowledging that his own preformation has empirical roots, Leibniz criticizes epigenesis for allowing the circumstances of empirical investigation to be brought to bear in the theoretical account of conception.
This confidence may be seen as flowing from Leibniz’s commitment to a version of the doctrine of rationes seminales, or seminal reasons. As Augustine explains this ancient doctrine:
The being that thus appears has already been wholly created in the texture as it were of the material elements, but only emerges when the opportunity presents itself. For as mothers are pregnant with unborn offspring, so the world itself is pregnant with the causes of unborn beings, which are not created in it except from that highest essence, where nothing is either born or dies, begins to be or ceases to be.29
Leibniz describes his particular version of preformation as agree- ing both with recent scientific discoveries as well as with religious doctrine:
And since even the smallest insects reproduce by propagation of their kind, one must conclude the same to be true for these little seminal animals, that is, that they themselves come from smaller seminal animals, and thus have originated only with the world. This agrees well with Holy Scripture, which suggests that there were seeds in the beginning.30
Although he does not mention Augustine when directly discussing his own theory of preformation, Leibniz does credit the Church Father with recognizing that animals have an indivisible soul, and thus an indestructible one: “It is true that Augustine suppos[ed], according to the prejudice common to all men, that beasts have a soul. . . . He believed that the souls of beasts were effectively spiritual and indivisible.”31
While Leibniz recognizes that Augustine was not opposed to the immortality of animals, he sees this belief as much more central in the thought of certain other philosophers: “I still maintain, as Plato already has and before him Pythagoras, who took this opinion from the Orient, that there is no soul that perishes, not even that of animals.”32 At the same time, however, Leibniz is intent to distinguish his own doctrine of pre-formation from that of Pythagoras, who famously believed in the trans-migration of souls. For Leibniz, the crucial difference between his view and Pythagoras’s is that Pythagoras argues only for the preexistence of souls, which might leave one body in order to enter another one, whereas Leibniz believes concretely that every creature is perpetually organically embodied, which is to say that there can be no transmigration, since no soul can ever depart entirely from the body in which it already finds itself. We will return to the differences between Leibniz’s theory and metempsychosis shortly.
The view that all things are contained in the world from creation finds a very early expression in Anaxagoras’s doctrine of logoi spermatikoi. For Anaxagoras, however, the notion primarily concerns the admixture of different elements within matter, and the potential extractability of any element from any other. In this respect, the Anaxagorean doctrine has important similarities to the theory of mixtures, briefly treated in chapter 1, that is associated with the school of “Latin pluralists” and that would, through Daniel Sennert’s theory of subordinate forms, perhaps come to have some influence on Leibniz’s mature theory of substance. In its Platonized, and subsequently Christianized and Latinized, form, the term logos spermatikos, if not the Anaxagorean doctrine itself, is transformed into ratio seminalis, a notion that is perhaps most closely associated with the Augustinian vision of the world, cited above, as “heavy” with the causes of things that have yet to come to be. However vivid the image of a pregnant world may be, Augustine gives no real account of the ontological status of things prior to the “proper opportunity for their appearance.” The fact that Augustine speaks in terms of “causes” suggests that he does not have actual preexisting things in mind at all, but is only claiming that God in his wisdom has created the world in such a way that each successive state of it follows in accordance with reason from its preceding states.
In sum, there are indeed ancient, philosophical precursors to Leibniz’s theory of preformation, and we must not see him as being entirely disingenuous when he claims that he would have found himself committed to it no matter what the state of technology and science in his era had been. But two scientific discoveries appear to inform Leibniz’s particular doctrine of preformation to the extent that we cannot talk about it as the same doctrine as, or as directly continuous with, ancient doctrines such as that of Augustine. One is insect metamorphosis, discovered by Jan Swam-merdam,33 and observed and written about extensively by Leeuwenhoek. The other is the discovery of microorganisms, which enabled Leibniz to pick out an actual bodily vehicle for the preexisting soul rather than letting the seed of the future animal or human remain a merely hypothetical entity or describing it vaguely as a seminal “cause” of a future, as-yet nonexistent entity. For a philosopher other than Leibniz, having such a vehicle might not be absolutely necessary, even if it would certainly help to make the philosophical theory of preformation much more empirically plausible. But for Leibniz, who believes for deep philosophical reasons that we have already considered that every created substance must be embodied, to the extent that embodiment is just a condition of the existence of substances, in order for preexistence to be a tenable theory this must be an embodied preexistence. In this respect, then, the discovery of preexisting seminal animalcules may be seen as helping to fill out an important part of Leibniz’s mature account of substance.34
In the previous two chapters, we saw how Leibniz’s otherwise Aristotelian conception of composite substance differs from that of Aristotle himself, as a result of the fact that Leibniz’s conception is formed in a period in which the microstructure of organic bodies has become a central subject of inquiry in science. Similarly, we may say that Leibniz’s theory of preformation differs from that of Augustine, in view of the fact that Leibniz’s theory is formed after “the seeds of things” had become a central subject of scientific inquiry, and indeed at precisely the same time that the true seeds of things were being positively identified.
We have seen that Leibniz’s generation theory derives not just from higher principles but also from his interest in empirical discoveries, not just from thinking but also from seeing. We have yet to consider what exactly was seen, or, perhaps better, what Leibniz and his contemporaries thought they were seeing. We should also consider, in this connection, how aware a theorist such as Leibniz may have been of the problems inherent in interpreting the newly observable rudiments of reproduction. That is, did Leibniz and his contemporaries believe that what they were observing spoke for itself, or did they sense that what the microscope had lain bare in their era could nonetheless serve to buttress any number of competing theories of generation? To adapt a phrase from Ian Hacking, did they ever ask themselves whether one really sees through a microscope?35
Caution in describing what one has observed was characteristic of most seventeenth-century thinkers seeking to corroborate their respective generation theories by means of empirical evidence. Most offered descriptions of embryonic development that were restricted only to comments on what they had seen.36 Some researchers, such as Malpighi, are hard to classify as either preformationists or as epigenesists since they seldom have anything to report beyond what they have observed, and nobody, in the earliest years of preformation, had ever observed either a spermatozoon or a mammalian ovum.
Interestingly, the same cautious limitation of claims to the observable had also been characteristic of Harvey’s epigenesis. He is a leading figure in the epigenesist tradition, insofar as his work on the chick embryo leads him to conclude that all animals develop into complex organisms from a uniform and undifferentiated primordium. Harvey believes that all living things, not just those born of oviparous mothers, start as eggs or egglike primordia, and that viviparous generation must be understood on analogy to the oviparous kind. Harvey identified the egg as the primordium of life and explained sexual reproduction and spontaneous generation in terms analogous to those of the reproduction of oviparous creatures. But he was unable to answer the question concerning the ultimate origins of the primordia of viviparous creatures, the discovery of the production of mammalian eggs in the ovaries having been made only two centuries later by Karl Ernst von Baer in 1827.37
Since Harvey could not find the mammalian egg, and since his method prevented him from commitment to any entity that cannot offer itself up to visual inspection, the English anatomist’s suggestion that all things come from eggs remained, from his own point of view, merely speculative. The research on mammalian ovaries that his work inspired—particularly that of Regnier de Graaf, immortalized in female anatomy itself as the discoverer of the Graafian follicles, and of Malpighi—would gradually pave the way for ovist preformation, as a result of two factors: (i) the development from a uniform primordium into a complex organism came to be seen as inexplicable within the increasingly dominant mechanist natural philosophy of the day; and (ii) simultaneously, the possibility of microscopic yet nevertheless complex entities came to be seen as plausible as a result of microscopy. Harvey is an epigenesist to the extent that he believes conception sets into motion the development of a new, complexly structured creature out of homogeneous materials, but the concern he takes, along with nearly all of his contemporaries, to find the primordia of these reveals that “epigenesis” and “preformation” need to be thought of as describing positions on a continuum rather than two radically distinct, mutually exclusive options. This is especially the case in view of the general interpretative caution of the theorists describing the observable evidence.
A similar caution to that exercised by Malpighi and Harvey in their interpretation of observable data is also characteristic of Leibniz, who tends to speak of preformation as merely probable and as the theory that best conforms to the observable evidence. Thus in a 1701 letter to A. C. Gackenholtz, on botanical method, Leibniz writes:
If this [account of plants] were attested by diligent observation, it would more solidly establish the acceptance of the doctrine of Kerckring and Leeuwenhoek, which always seemed to me most probable: namely, that something subtle, which was already organic, and which could be denoted by the name of “plant” or “animal,” reaches the female’s eggs from the male seed, and from here, as if transformed in its proper soil, develops into something more elaborate, a fetus, through [a process] that may be called generation.38
So “generation” is probably, though not certainly, just development into something more elaborate; it does not involve an addition to the list of existents. One may wonder why Leibniz, who seems compelled in view of his deep philosophical commitment to the indestructibility of corporeal substances to hold that there can be no true generation, would nonetheless use the cautious language of probability when discussing the empirical study of generation. The answer to this question probably has to do with a concern that no amount of empirical observation could resolve the deeper questions about generation, that generation science can only speak in terms of probability, even if one’s metaphysical principles might favor one particular generation theory over another. Interestingly, then, Leibniz is willing to express certain commitment to preformation insofar as he is speaking of it as a doctrine that flows from higher, a priori principles, but only partial commitment to it insofar as he is treating it as an empirically grounded account of observable evidence. Leibniz’s probabilism in the latter case seems to have to do with a more general commitment to the view that any empirical theory is at best probably true.
The observable evidence appears to have been interpreted very differently by different theorists depending upon the sex of the animal body that was the primary focus of investigation. This difference of interpretation gave rise to two opposed camps among the preformationists: the ovists maintaining that the primordium of future organisms preexists as an egg in the mother, and the animalculists believing that it preexists as a spermatozoon.
While Leibniz’s preformation might derive from higher principles, one central detail of his version of the theory would only become possible in the mid-1670s as a result of the discovery of spermatozoa. Leibniz’s mature version of the doctrine of preformation would come to hold that all corporeal substances came into existence at the creation of the universe, as living, embodied entities, and that, at least in the case of all vegetable and animal corporeal substances, these exist prior to what appears to be their coming-into-being in the form of seeds or spermatozoa.39 Although Leibniz’s chosen primordium, the spermatozoon, is vastly smaller than the mammalian ovum, Harvey’s preferred source of the future animal, historical contingencies would promote the former beyond the status of mere theoretical entity and into the category of observables much earlier. In this respect, animalcular or spermatozoid preformationism was empirically a more well grounded theory in the seventeenth century.
We have already seen that from the very beginning of his career, the young Leibniz was interested in taking into account the findings of microscopical research in his reflections on the nature of generation. But the crucial piece of empirical information that would help him to develop his mature theory of generation would come only in 1676, with his exposure to the microscopic research of Antoni van Leeuwenhoek.
Catherine Wilson suggests that it was Leibniz’s 1675 reading of Malebranche’s Search after Truth that “showed [Leibniz] how the microscope fitted in with metaphysics. Malebranche saw preformation as a necessary correction to strict Cartesianism, which was epigenetical.”40 However, as we saw in the previous chapter, Leibniz’s interest in microscopy reaches back at least as early as 1669, even if it was not until 1676, one year after reading Malebranche, that Leibniz looked through a microscope for the first time, on a visit to Leeuwenhoek in Delft. His conviction as to the fit between microscopy and metaphysics was evidently only strengthened, rather than conceived, at this moment. In time, Leibniz would come to believe that “a Leeuwenhoek” is preferable to “a Descartes,” not just because Leibniz will prefer preformation to epigenesis, but also, more importantly, because it is Leeuwenhoek’s cautious observation, as opposed to Descartes’ wild speculation, that shows the way to the correct account of a problem such as the nature of generation.
The year 1676 was an eventful one for the still reasonably young Leibniz. Having just finished four years of intensive mathematical study under Christiaan Huygens in Paris, in early October he set out on a trip to England and the following month traveled on to Holland. Among the luminaries he had a chance to meet in England were the Royal Society’s Henry Oldenburg and John Collins. After arriving in Amsterdam in mid-November, Leibniz then made the acquaintance of Swammerdam, the discoverer of insect metamorphosis, and also met with the controversial Spinoza. He managed to squeeze in yet another appointment during the Dutch leg of the journey, one that has, no doubt, prompted a good deal less speculation throughout the centuries than did his visit to the author of the Ethics. This other visit was to Leeuwenhoek, who honored Leibniz by giving him a brief demonstration of the uses of the microscope, and, we may speculate, briefing Leibniz on what had been his own most remarkable microscopic discovery to date: the spermatozoon.41 This visit was motivated by a belief—one that can be traced back as early as 1669, and was evidently shared by Oldenburg, whom Leibniz had just visited in London, as well as by Huygens, who, again, had been Leibniz’s mentor for the past four years—that the findings of the microscopists are of tremendous relevance to our understanding of the basic questions of metaphysics. Evidently, Leibniz was in no way let down in this belief by the visit to Leeuwenhoek.
A year or two prior to Leibniz’s visit, according to Leeuwenhoek’s recollections decades later, the Dutch microscopist had carried out research on the “animalcula in semine marium” (animalcules in the seed of males) of a number of animal species, including humans. According to Leeuwenhoek’s variety of preformation, all future adult humans are contained within the testicles of their male ancestors, and the animal embryo is enveloped in microscopic form within the head of the spermatozoon. As Leeuwenhoek insists in a letter of 1678, “it is exclusively the male seed that forms the fetus and . . . all that the woman may contribute only serves to receive the seed and feed it.”42 Leeuwenhoek will later write in 1700, regarding his priority dispute with Nicolaas Hartsoeker over the discovery of spermatozoa and the development of the theory of animalcular preformation, that he, Leeuwenhoek, “did not only give the Royal Society an account of [spermatozoid preformation], by my letter of Novemb. 1677, but even 3 or 4 years before, at the request of Mr. Oldenburg, I had made an inquiry into those matters.”43 We can infer from this letter, unless Leeuwenhoek’s memory is in error, that the Dutch microscopist was actively researching the function of spermatozoa and developing his own theory of animalcular preformation around 1674 or 1675, thus roughly one or two years before Leibniz’s first visit to Leeuwenhoek in Holland in 1676.
Writing in April 1679 to Nehemiah Grew, Leeuwenhoek proudly takes credit for the solution of the mystery surrounding the function of the testicles:44
As a result of my observations . . . I have no doubt but that yourself and the learned Philosophers will agree with me in stating that the testicles have been made for no other purpose than to furnish the little animals in them, and to keep them till they are ejected. . . . Also, those who have always tried to maintain that the animalcules were the product of putrefaction and did not serve for procreation will be defeated.45
Of course, one might believe in the importance of seminal vermicules for conception as part of that mysterious efficient cause of the epigenesists, without necessarily subscribing to the view that the human embryo is preformed in these vermicules. This is clearly not Leeuwenhoek’s position. He maintains, like Leibniz, that “all creatures that are endowed with a moving or living Soul, depend upon . . . the moving or living animals that were made in the male seeds, in the beginning of creation.”46
Leeuwenhoek offers a number of arguments for his view that the spermatozoa are but future animals in an extremely miniature form. The first of these arguments is based on his direct observation of plants enveloped within vegetable seeds, and the claim that of necessity the same envelopment must occur in animals. This claim rests on the assumption that animal “seed” functions exactly as does the seed of an apple tree. Leeuwenhoek notes that in the seed of an apple we can actually see the leaves and trunk in an extremely small, enveloped form. Why may we not then assert, he asks, “that in an Animalcule from the Male seed [het Mannelijk zaad] a whole Human being is enveloped and that all Animalcules from the Male sperm derive from the first Man that was created?”47 In another letter to the Royal Society, Leeuwenhoek argues that the best evidence we have for animalcular preformation, in light of the difficulty of actually observing spermatozoa, comes from the observation of other microorganisms:
It will seem strange to many, that cannot comprehend, how in an Animal of the Masculine Seed, that is so incomprehensively small, so great a Secret, as a Body of a Man doth comprehend, can be Locked up. But if we remember that there are Living Creatures in Waters, that we have many times seen come before our Eyes, that being of a Roundish Body, were no thicker then the thinest end of a Tayl of an Animal in the Masculine Seed.48
In a letter of August 1688, Leeuwenhoek cites the empirical evidence of the envelopment of plants within seeds and goes on to claim that it is in keeping with God’s ordering of the universe at creation that animals be similarly enveloped. “God,” he writes,
Lord and Omniscient Maker of the Universe, makes no new Creatures [nieuwe Schepsels], but . . . He, Lord and Creator, has so ordered and made it from the beginning, that all well-made or full-grown Seeds of plants (although it may remain hidden from our eyes) are already created therein. . . . Which, thus happening in Plants, I assert, must necessarily take place also in the Male Seeds of all Animals [de Mannelijke Zaaden van alle Dieren].49
Leeuwenhoek explicitly rejects the caricatured view according to which the human fetus might be detected with a powerful microscope in the head of a spermatozoon. He sees this as a primitive conception of the preformationist theory, and ridicules the idea that human semen might be “full of small babies [vol van kleijne kinderkens].”50 Leeuwenhoek argues instead that though the future human’s organic body does actually exist in the spermatozoon, it must undergo a metamorphosis, just as do amphibians and insects, in order to become recognizable as a human baby: “Just as we have no reason to say that some worms, while they are still swimming in water, are flying creatures, though creatures with wings will eventually emerge from them . . . it would be equally wrong to assert that the little worms in the human sperm are small babies, even though a child is formed from such a small worm.”51
Spermatozoid preformation would gain quite a bit of popularity by the end of the century. Nicolas Andry, for instance, to whom we were introduced in the previous chapter, writes in his De la génération des vers dans le corps de l’homme of 1700:
It thus does not appear unreasonable to think that in one single spermatic worm there is an infinity of organized bodies capable of producing an infinity of animals: so that according to this thought, which could only appear bizarre to those who measure the wonders of the infinite power of God according to the ideas of their senses and of their imagination; one could say that in one single spermatic worm, there would be organized bodies capable of producing fetuses and children, for infinite centuries, always smaller and smaller in relation to one another.52
We saw in the previous chapter that Leibniz does not begin to explicitly identify corporeal substances as worms until the latter half of the 1690s, within a few years of Andry’s treatise on worms in the human body. Claims concerning the status of spermatozoa as the vehicles of organic bodies, similarly, do not begin appearing in an explicit and unequivocal form in Leibniz’s writings until 1695, with the Système nouveau. Duchesneau holds that as late as 1686 “Leibniz did not even exclude the possibility of an explanation of the formation of living beings by means of a mechanical epigenesis—or, more exactly, he had not yet decided between epigenesis and preformation.”53 Duchesneau is likely referring to Leibniz’s first acknowledgment of Leeuwenhoek’s theory of preformation in the correspondence with Antoine Arnauld. In this correspondence Leibniz first begins to claim that the organism preexists and that it continues existing after what is vulgarly called death. Characteristically for this correspondence, Leibniz invokes empirical microscopy somewhat tentatively as justification for his philosophical claims. As he writes in a letter of October 1687, evidently recalling his visits to Delft of more than a decade earlier:
I learned some time ago that Monsieur Leeuwenhoek holds opinions very close to mine, in that he believes that even the largest animals arise through a kind of transformation. I do not venture either to approve or to reject the details of his opinion.54
There is, then, a period of at least ten years during which Leibniz likely knew of the discovery of spermatozoa without incorporating this knowledge in any significant way into his philosophy. There is, moreover, a period of around twenty years, from the visit to Holland to the publication of the Système nouveau,55 between Leibniz’s probable first discussions of the theory of spermatozoid preformation and his explicit localization of the preexisting soul within the spermatozoon in 1695. By the time of the Principes de la nature et de la grace of 1714, Leibniz is so convinced of the spermatozoid preexistence of all creatures, including humans, that he casually refers to humans in the state of preexistence as “human spermatic animals [des animaux spermatiques humains].”56
Duchesneau has also expressed some hesitance to describe Leibniz as a wholehearted animalcular preformationist. He writes that “Leibniz would draw some support for his own “philosophical position” on the envelopment and development of organisms from the then current doctrines and observations on germ preformation. But he generally avoided siding radically with either the ovist or the animalculist theory.” Duchesneau concludes: “I would be reluctant to assimilate his position to Leeuwenhoek’s whom he mostly admires for his technical skill, not for his theoretical views.”57 Yet perhaps when Leibniz says he prefers “a Leeuwenhoek” to “a Cartesian,” we may take this to mean that, in his view, it is precisely through the sort of technical skill that Leeuwenhoek has mastered that one arrives at the correct theoretical views. Duchesneau takes the correspondence with Louis Bourguet from the years 1709 to 1716 to be most representative of Leibniz’s views on the matter of preformation. In this correspondence, Leibniz openly refuses to take sides between the ovists and the spermists. In one letter, he is particularly careful to point out that he is not opposed to the claims of the physician Antonio Vallisneri, who had denied Leeuwenhoek’s theory:
M. Camerarius of Tübingen believed that the grain was like the ovary, and the pollen (though in the same plant) like the sperm of the male. But if this were true, the question would still remain whether the basis of the transformation of the preformed living being is in the ovary, following M. Vallisneri, or in the sperm, following Leeuwenhoek. For I maintain that there must always be a preformed living being, be it a plant or an animal, that is the basis of the transformation, and that this is the dominant Monad itself: no one is better suited to clarify this doubt than M. Vallisneri.58
Significantly however, as we learn from a footnote in the Gerhardt edition of this letter, Leibniz’s original draft did not contain the above passage, and instead we find Leibniz more sympathetic to the animalculists than to Vallisneri:
Up until now the Hypothesis of seminal Animals has seemed to me the most plausible. It also seemed attractive to M. Huygens, to M. Hartsoeker, and to others. What I say is meant neither to contradict M. Vallisneri, nor to prevent his judgment, which in my view has much weight, but to motivate him to clarify this important matter.59
Considering the agreement of this deleted passage of the letter to Bourguet with Leibniz’s mature-period claims elsewhere concerning the embodiment of the preformed animal in the spermatozoon,60 it appears likely that the view expressed in the correspondence with Bourguet is not the best expression of Leibniz’s actual views concerning preformation, but instead that what Leibniz writes to Bourguet might rather be motivated by a concern not to offend a particular correspondent with what he would otherwise have wanted to write (and, even in this case, did write at first). As we have already seen, elsewhere Leibniz is much more open about his support for Leeuwenhoek’s theory of preformation than he is in the Bourguet correspondence.
Leibniz sometimes speaks of Leeuwenhoek’s work as mere confirmation of the conclusions to which reason would have led us without the aid of experimental science:
Besides that which . . . M. Leeuwenhoek has observed of this, one could say that reason leads us there just as much as does experience, because there is nothing in mechanics that could make from an unformed mass a body endowed with an infinite number of organs, such as that of the animal.61
But as we have already seen, Leibniz also frequently relies on Leeuwenhoek’s discoveries as one source of empirical grounding of a metaphysical theory that might otherwise appear rather outlandish. In short, Leeuwenhoek’s work, though Leibniz might not have wanted to admit it, was a crucial support without which Leibniz likely would not have dared to make the claims he did. One detects a deep admiration for Leeuwenhoek on the part of Leibniz, almost a feeling of solidarity in an ideological struggle. In the Entretiens de Philarète et d’Ariste of around 1712–14, Leibniz, probably only half playfully, writes: “According to this Hypothesis [of ovist preformation] the male would hardly do anything more than the rain. But Mr. Leeuwenhoek has rehabilitated the masculine gender and the other sex has been degraded in turn.”62
In sum, for Leibniz, right up through his most mature writings, it is necessary that no substance ever come into being, and that every substance always exist in an organically preformed body. From 1676 until very late, the spermatozoon discovered by Leeuwenhoek serves as the most likely candidate for this role of preexisting soul-bearer.
For Duchesneau, the fact that the preexisting entity bears no clear resemblance to the eventual creature is enough to feed a doubt as to whether Leibniz was really a preformationist at all, as even the great epigenesist Harvey would insist that every creature comes from some preexisting primordium. For our understanding of Leibniz’s particular variety of preformation here, we need to turn our attention not to the empirical discoveries of Leeuwenhoek but to those of his countryman, Jan Swammerdam, whose name Leibniz consistently associates with the discovery of insect metamorphosis.63
For Leibniz, what is commonly known as the generation of animals is not just an augmentation of the preexisting organism but also “une grande transformation,” as a result of which the spermatozoon becomes “un animal d’une autre espece.”64 It is only through this “revestement nouveau”65 that the preformed creature is given “the means of nourishing itself and of growth, in order to pass into a larger theater [le moyen de se nourrir et de s’aggrandir, pour passer sur un plus grand theatre].”66 In the Nouveaux essais, Leibniz writes tellingly of ancient theories of transmigration: “I do not believe that a person who would be certain that the soul of Heliogabalus existed in a pig wanted to say that this pig was a man, and the same man as Heliogabalus.”67 Individual identity runs more deeply than species membership, whether the individual in question is an insect that moves from the larval to the adult stage, or whether it is a man reincarnated as a pig (though Leibniz does not really believe in the latter possibility).
One example of species change that Leibniz does affirm is that of a standard-grade, preexisting monad into a human spirit at conception. For Leibniz, the difference between a human soul and an animal soul is that the former is a mind, a mens or esprit, and this means most importantly that it is capable of reasoning and is a suitable subject of reward and punishment. In some short works of the mid-1680s,68 Leibniz worries that a preexisting spermatozoid human could not be considered human prior to conception, since if this were the case the vast majority of human lives would be wasted at conception, which is contrary to God’s wisdom and grace. Indeed, the discovery of spermatozoa, on Leeuwenhoek’s interpretation, meant that the biblical injunction against wasting seed would be literally impossible to follow: even in proper, marital intercourse, most of the seed comes to no use, and thus in this respect marital intercourse is scarcely different, morally, from onanism. But neither, for Leibniz, could a preexisting spermatozoon become a human being at conception; that is, neither could it somehow be promoted from the status of mere corporeal substance to the status of a human being with a mind. For, in Leibniz’s view, in the texts of the mid-1680s, this would not constitute a transformation but a discontinuous rupture, the destruction of one entity and the creation of another. Thus, at least in this period, Leibniz does wish to excuse human beings from the process of preformation, the reality of which he otherwise accepts, and instead see the conception of a human being as a miraculous creation involving the intervention of God.
Leibniz would gradually come to change this view, however. In the Système nouveau of 1695, it is not clear that Leibniz wants to exclude humans from the process of preformation. In this work, Leibniz praises Swammerdam, Malpighi, and Leeuwenhoek as “the best observers of our time,” who have “made it easier for me to admit that animals and all other organized substances have no beginning . . . and that their apparent generation is only a development, a kind of augmentation.”69 He does distinguish sharply between the ungenerability and incorruptibility of ordinary forms or brute souls on the one hand and human minds or rational souls on the other, which “have particular laws,” placing them “above the upheavals of matter.”70 He maintains that “rational souls follow much higher laws, and are exempt from anything that might make them lose the quality of being citizens of the society of minds.”71 But nowhere in this text does he explicitly wish to exempt human individuals from embodied pre- and postexistence, and he seems to be careful to distinguish humans from animals in important respects while nonetheless not denying that pre- and postformation must hold of all created substances, even of human beings.
In the Principes de la nature et de la grace of 1714, in any case, there can be little question where Leibniz stands; there, as we have seen, he is so convinced of the spermatozoid preexistence of all animals, including humans, that he casually refers to humans in the state of preexistence as “human spermatic animals.”72 The broader context of this surprising description runs as follows:
There are small animals in the seeds of large ones, which, through conception, assume new vestments that they appropriate for themselves, which give them the means to nourish themselves and grow in order to pass to a larger theater and to bring about the propagation of the large animal. It is true that the souls of the human spermatic animals are not rational and do not become rational until conception settles that these animals will have a human nature.73
So it is settled: no human souls are lost in ejaculation, since the soul is not human until conception; but transformation of a preexisting spermatozoid soul into a human soul does not consist, as Leibniz had earlier worried it would, in the simple destruction of one soul and the simultaneous creation of another. It is, rather, a transformation like any other, for instance, like that of a caterpillar into a butterfly.
Daniel C. Fouke has provided a clear analysis of the problem of how bare-level monads are promoted to the rank of rational beings in the course of human reproduction.74 Fouke rightly argues that Leibniz is forced to deviate from his general account of the successive states of substances as unfolding spontaneously from its individual nature, describing the promotion to rationality as instead a rare instance of divine intervention in the natural order. One point that Fouke does not emphasize is that this deviation would not have been necessary had Leibniz not been fundamentally committed not just to some sort of preexistence but also specifically to preexistence in an organically embodied form. We have been arguing that the identification of the future human, like other future animals, with the spermatozoon was an important part of the need Leibniz perceived to develop an account of promotion to rationality at some moment after conception. Clearly, this need would not have been felt in the same way if the future human were conceived as a disembodied soul in a prebirth baby heaven. Spermatozoa as the bodily vehicle of future humans were both a solution to a problem Leibniz faced—given that every substance must be constantly embodied, where is the body of the preexisting human?—as well as the source of new problems, such as the condition of the future human prior to conception.
Theologically motivated exceptionalism vis-à-vis human beings should not diminish the significance of the impact of a theory such as preformation on Leibniz’s thought. The great lengths to which he goes to explain how humans might be exempted from preformation drives home the point that, for him, in the ordinary course of nature nothing is generated ex nihilo; in order for humans to be so generated, it must be conceded that their generation violates this ordinary course. A high price to pay for theological correctness.
Leibniz, as already mentioned, frequently juxtaposes his own doctrine of metamorphosis to the ancient belief in reincarnation. What appears to us as death, he believes, is a mere transformation of the animal body, but not a departure of the soul from the body, into either a permanently disembodied state or into another body through metempsychosis:
Thus not only souls, but even animals are ungenerable and imperishable, they are only developed, enveloped, returned, depleted, transformed; souls never leave their bodies, and never pass from one body to another body that is to them entirely new. There is thus no metempsychosis, but there is metamorphosis.75
Elsewhere Leibniz describes his theory of metamorphosis as “metensomatosis.”76 He explains that this term is a synonym of “transformation,” which is of course the Latinized form of “metamorphosis.” Here we use the term “metamorphosis” because it provides the clearest contrast with the original Pythagorean and Platonic doctrine, and because it suggests an analogy to processes in the natural world. It is, however, important to bear in mind that the term “metamorphosis” did not resonate in Leibniz’s time as primarily an entomological term any more than the Latinate “transformation” does today. But this does not diminish the fact that it was an analogy to processes in the insect kingdom that Leibniz intended when employing the term “metamorphosis” in his account of what happens to the organism after death.
In the piece just cited in which Leibniz speaks of “metensomatosis,” the De natura mentis et corporis, dated by the editors of the Academy Edition to between 1683 and 1686, we encounter the only occurrence in Leibniz’s writing of the term “metensomatosis,” used as a synonym of the generally preferred “metamorphosis.” Here Leibniz writes:
If we imagine some animal similar to a butterfly that is made out of a worm, and prior to that was of such a kind that the worm, in turn, was made from something else that was up until then very small, and thus again back to the beginning of the world, and where the progress of the animal into a worm is extinguished, or is pushed back into some non-sensible animalcule (nor am I speaking of those worms, that arise per accidens from members), with the soul likewise always remaining in all of them, there will indeed not be mere Metempsychosis in this anima, but indeed metensomatosis or transformation, and indeed it will not so much always be the soul [that remains], but likewise always the corporeal substance.77
Metensomatosis, then, is for the immortal corporeal substance what metempsychosis is for the immortal soul. This term is evidently not Leibniz’s invention. In Goclenius’s Lexicon philosophicum of 1613 we find an entry for “Metensomatosis, that is, transcorporation, for example, when the soul migrates from the human body into that of a cat.”78 Goclenius identifies this doctrine with the teachings of Zoroaster. As with the concept of the “organic,” then, Leibniz is taking an already familiar, if obscure, term and radically reconceiving its meaning. “Organic” had been a synonym of “mechanical,” and “metensomatosis” evidently a synonym of “metempsychosis”; Leibniz appropriates both of these terms and renders them not so much as synonyms of their respective related notions but as special cases.
In a letter to Arnauld of April 30, 1687, Leibniz explicitly mentions Leeuwenhoek’s experiments with pepper water to corroborate his commitment to the metensomatosis of animal souls (described here as “metaschematismus”):
We find that there is a prodigious quantity of animals in a drop of water imbued with pepper. . . . Now if these animals have souls, we would have to say of their souls what we can probably say of the animals themselves, namely, that they were already alive from the creation of the world. . . . The ancients were mistaken in introducing the transmigration of souls instead of the transformations of the same animal which always preserves the same soul; they put metempsychoses pro metaschematismus.79
As preformation satisfies both biblical and empirical demands, so, too, does the metamorphosis of the creature, including the human being, after death. Leibniz believes that the bodies of the blessed must accompany them into the afterlife and so requires a theory that will not permit the soul to become disembodied: “For why cannot the soul always retain a subtle body organized after its own manner, which could even someday resume the form of its visible body in the resurrection, since a glorified body is ascribed to the blessed?”80 Leibniz writes that this doctrine conforms not only with religious teaching but also with the order of nature established through experience, “for the observations of very capable observers have convinced us that animals do not begin when they are popularly believed to begin. . . . And both order and reason demand that what has existed since the beginning should no more have an end.”81 If the origins of Leibniz’s spermatozoid preformation can be traced in part back to Leeuwenhoek, it is Swammerdam who deserves the credit for the doctrine of metamorphosis. Thus Leibniz writes that he is indebted to Swammerdam for the discovery that “the silkworm and the butterfly are the same animal . . . the parts of the butterfly are already enveloped in the caterpillar.”82
In contrast to his own doctrine of metamorphosis, Leibniz denounces metempsychosis as “le Dogme de Pythagore,” and claims that he is “fort éloigné” from the Pythagorean view. But his explanation for why it is correct to view his own predictions concerning the afterlife as greatly distanced from that of Pythagoras amounts to an explanation of the great affinity of the two doctrines, the only difference being that Leibniz’s version involves a physical as well as a spiritual aspect: “I am very far removed,” he writes, “because I believe that not only the soul, but the same animal subsists as well.”83 In other words, it is the constant organic embodiment of the animal that ensures that its soul can never migrate from one body to another.
This constant organic embodiment, moreover, is comprehended by Leibniz as a consequence of preestablished harmony. In short, metamorphosis as opposed to metempsychosis is guaranteed by organic preformation. Preformation, moreover, is in turn guaranteed by preestablished harmony. Let us turn now to the connection between these latter two doctrines.
Divine preformation is not just a necessary consequence of Leibniz’s model of infinitely complex organic bodies but also of his adoption of the doctrine of preestablished harmony. Because there must always be a body “proportioned to the perceptions” of every simple being, and because simple beings cannot come into or go out of existence, it follows that every simple being has always existed in an organically embodied form. The preestablishment of harmonious perceptions and the preformation of organic bodies are of a pair.
Divine preformation, as we have already seen in chapter 3, plays a crucial role in Leibniz’s rejection of Cudworth’s immaterial plastic natures. In her summary of Leibniz’s metaphysics, Cudworth’s daughter, Damaris Masham, ascribes to Leibniz what she calls the “Principle of Uniformitie,” correctly discerning the connectedness of the preformation on the one hand and the infinite complexity of the organic body on the other:
All these Simple Beings you thinc have; always will have; and ever since they existed have had Organick Bodys, proportion’d to theire Perception. So that not onely after Death the Soul dos remain: but even the Animal also. . . . The same Principle of Uniformitie in the works of Nature . . . led you also to your Systeme of the Harmonie preestablish’d betweene substances.84
Although for the reasons just described there would appear to be a natural, perhaps necessary, fit between preformation and preestablished harmony, curiously it is in Malebranche’s Search after Truth of 1676 that we find the most elaborate philosophical account of preformation. Male-branche is an occasionalist, and thus, some commentators argue, we should see the theory of preformation as best suited to the exigencies of this theory of causation. Malebranche writes:
We ought to accept . . . that the bodies of every man and beast born till the end of time were perhaps produced at the creation of the world: I mean that the females of the original animals may have been created along with all those of the same species that they have begotten and that are to be begotten in the future.85
To be sure, preformation is compatible with occasionalism, but there is no good reason to believe that acceptance of the former doctrine would impel one to accept the latter. Leibniz did not see any reason either. Indeed, if Leibniz were to draw an association between occasionalism and any particular theory of generation, he would probably see occasionalism, which posits the continuous intervention of God in the world, as most similar to the theory of epigenesis, which posits a continuous creation of new beings by God. Cartesian epigenesis, underlain as it is by an ontology on which animals are nothing more than machines, would not require such perpetual recourse to miracles, since the formation of an animal-machine is not in any sense a coming-into-being of a new substance, but only a new articulation of matter. For Leibniz, however, for whom animals are always simultaneously machines and substances, a universal epigenesis would be offensive for precisely the same reason Malebranche’s occasionalism is: it would require that God see to individual comings-into-being directly, when in fact these can be accounted for nonmiraculously in terms of the this-worldly unfolding of substances, all of which came into being simultaneously in the singular super-miracle of the Creation.
While an occasionalist such as Malebranche may coherently defend a theory of preformation, it is only a defender of preestablished mind-body concomitance86 and preestablished intermonadic harmony who must, at risk of incoherence, defend such a view: if there is no preformed body, then it makes no sense to hold that the soul and body are in preestablished harmony with each other. Although Malebranche and Leibniz have differing accounts of causation, both see preformation as a means of remaining faithful mechanists in their respective accounts of animal generation.87
In sum, Malebranche defends preformation not because he is an occasionalist and sees occasionalism and preformation as of a pair, but because he is a mechanist and sees mechanism as incompatible with epigenesis. Leibniz, in turn, defends preformation both because he is a mechanist and because he is committed to the doctrine of preestablished harmony. Leibniz, like Malebranche, sees epigenesis as inexplicable in mechanical terms, and, unlike Malebranche, sees preformation as a necessary part of the doctrine of mind-body concomitance or preestablished harmony. On one possible understanding, preformation and preestablished harmony might be thought to be in tension with each other, to the extent that preformation might seem to depend upon some robustly conceived unity between an organic body and an immaterial principle, while preestablished harmony drives a wedge between body and the immaterial principle. But preformation requires no more union than what preestablished harmony affords: organic embodiment is a condition sine qua non of the existence of a substance, and even if this embodiment may, in the end, be traced back to a condition of simple substances that falls short of union with an independently existing body, nonetheless this does not mean that the immaterial principle’s connection with the organic body is contingent or temporary. Inseparability does not imply metaphysical union.
In Leibniz’s view the doctrines of preformation and of preestablished harmony are closely connected, insofar as the harmony of a soul with a body since the creation requires that there be such a body since the creation. It speaks for this close connection of the doctrine of animalcular preformation with the doctrine of preestablished harmony that Leeuwenhoek attempts to justify his commitment to preformation in a way that closely anticipates Leibniz’s reasoning in defense of the doctrine of preestablished harmony. In a letter published in the Philosophical Transactions of August 1699, Leeuwenhoek presents a typical statement of his conviction that all humans existed as spermatozoa within the body of Adam:
Now if we know which way the fish do increase, that it is not done but by intermixing of the Male and Female Seeds, and likewise we do know the great Mistery that is included in the small Seed of an Apple, why might not we then assert that in an Animal of the Masculine Seed of a Man, is locked up a whole Man, and that the Animals of the Seed are all descending from the first Created Man.
Leeuwenhoek is insistent on finding an interpretation of empirical evidence that does not require appeal to what, in his view, could only be explained by perpetual divine intervention, which Leeuwenhoek would consider miraculous:
It being then that hitherto, nothing at all is come before me that can make me the least Scruple, to induce me to recede from my former opinion, and to receive an opinion to believe, or hold, that Animals should come forth of themselves, therefore I still remain of this my opinion, that out of the Animals of Masculine Seeds, come forth Animals of the same kind as they were Created in the beginning, and that as hitherto no truer Position is left. For if Animals could be born of themselves, which I should reckon to be a Miracle, then must not only every Minute, but every Second, Millions of Miracles be done, which is an opinion not to be received.88
For Leibniz, similarly, preestablished harmony is the only account of the relationship between mind and body that manages to avoid appealing to miracles. Leibniz repeats throughout his mature thought that the doctrine of preestablished harmony is superior to other competing doctrines because it
excludes every concept of miracle from purely natural actions and makes things run their course in a regulated and intelligible manner. Instead of this, the common system has recourse to absolutely inexplicable influences, while in the system of occasional causes God is compelled at every moment, by a kind of general law and as if by compact, to change the natural course of the thought of the soul to adapt them to the impressions of the body and to interfere with the natural course of bodily movements in accordance with the volitions of the soul. This can only be explained by a perpetual miracle, whereas I explain the whole intelligently by the natures which God has established in things.89
The fact that Leeuwenhoek is interested only in refuting epigenesis, while Leibniz’s primary interest is in refuting occasionalism, does not render the structural similarity of their respective arguments any less interesting. Leeuwenhoek is concerned to avoid the coming into existence of new animals that is posited by nonpreformationist accounts of generation. For Leeuwenhoek, such accounts include both epigenesis and spontaneous generation. Epigenetic generation, on Leeuwenhoek’s view, can be seen as a special case of spontaneous generation: the new life is “spontaneously” brought into existence through sexual intercourse. How spontaneous generation of rats from bilge or eels from mud could occur is for Leeuwenhoek no greater a mystery than an entirely new coming-into-being that results from the entry of sperm into the womb. In the absence of a preformationist account, Leeuwenhoek believes that there are two possible ways in which one might attempt to explain animal generation: it can be explained either as the result of some inexplicable interaction of the male’s and the female’s material contributions, or it can be explained as the result of a direct intervention on the part of God.
Consider, now, Leibniz’s explanation of the various possible relations between mind and body. He writes in the “Second Explanation” of the Système nouveau that the agreement or sympathy between the soul and the body will come about in one of three ways.
Now, for comparison, let us formulate in Leibnizian terms Leeuwenhoek’s account of what he sees as the three possible ways in which generation could occur:
On the basis of this comparison, we may discern a striking analogy between each of the prominent early modern theories of cause and a corresponding theory of animal generation:
Theory of cause |
Theory of generation |
Cartesian interactionism |
Epigenesis |
Occasionalism |
Spontaneous generation |
Preestablished harmony |
Preformation |
Of course, Malebranche is both a preformationist and an occasionalist, so it would be a mistake to suppose that there is something about preformation that forces one to prefer preestablished harmony. Nor is Malebranche a great defender of spontaneous generation. The point is simply that if we switch “generation” and “causation,” we may say that in a sense Leibniz sees Malebranche’s theory of causation as a sort of perpetual recourse to miracle or divine conception in the same way that one might see commitment to the reality of spontaneous generation as involving commitment to ubiquitous, daily miracles, wherever slime forms into a frog or dust into a mouse. This is not to say that preformation and preestablished harmony must go together, but only that Leibniz’s reasoning in his rejection of competing theories of causation looks a great deal like Leeuwenhoek’s reasoning in his rejection of competing theories of generation.
In Leeuwenhoek’s adoption of preformation and Leibniz’s of preformation and preestablished harmony, each had to consider three options. Either the current state of things is the result of an inexplicable influence of matter on other matter, or it is the result of God’s perpetual intervention, or it is the result of the initial conditions set up by God at the beginning of the world. Leeuwenhoek saw the last option as most satisfactory in relation to the question of the origins of organisms; Leibniz saw the last option as the best explanation of both the origin of organisms and of the relation of the organism’s body to its soul. Since Leibniz appears to have been aware of Leeuwenhoek’s argument for preformation before he himself accepted preformation and before he developed his theory of mind-body concomitance (again, Leeuwenhoek’s theory of preformation was in place by 1675, Leibniz met with Leeuwenhoek in 1676 and first ventured a preformationist theory in the correspondence with Arnauld a full decade later), it would not be careless to hypothesize an influence of the microscopist on the philosopher, both in the philosopher’s acceptance of the doctrine of spermatozoid preformation and in some of his reasoning concerning the doctrine of preestablished harmony.
Leeuwenhoek directs most of his argumentative energy in defense of preformation against “Corn-dealers; Bakers, Millers, and those who cannot see past the end of their nose,”94 obviously not people to whom Leibniz devoted a great deal of his philosophical attention. But the humble level on which Leeuwenhoek’s argumentation was conducted should obscure neither the fact that his preformationist claim is exactly the same as Leibniz’s—namely, that the spermatozoon is the bearer of the human soul—nor the fact that the two thinkers’ arguments for this theory are very similar. Moreover, considering the similarity between Leeuwenhoek’s argument for preformation and Leibniz’s for preestablished harmony, perhaps these two doctrines should be conceived as more closely intertwined within Leibniz’s thought than they have usually been thought to be.
Leibniz, as we have seen, leaves no room for spontaneity in the generation of animals. For an epigenesist such as Descartes, in the end sexual generation is on an ontological par with the purportedly spontaneous generation of bees from rotting carcasses or of frogs from pond scum: both proceed according to “minor laws” as a result of the rearrangement of matter. For Leibniz, in contrast, the possibility of a new creature coming into existence as a result of such a rearrangement appears absurd: insofar as an animal is organically embodied, and insofar as this organic body is infinitely complex (as was discussed at length in chapters 3 and 4), it follows that no entirely new creature, with an entirely new organic body, can ever come into existence. Neither biological parents nor Mother Nature is capable of bringing forth a body that would require infinitely many steps for its production, and so the only other possibility is that that body has always existed in harmonious concomitance with the soul of which it is the body. Indeed, since no substance has any influence on any other and every state of every body unfolds only from its own prior states, it stands to reason that Leibniz could not believe that the most significant change-of-state of an animal, its coming-into-being, would be something that could be brought about, in the case of sexual reproduction, by epigenetic causes, or, in the case of spontaneous generation, by natural causes such as the proper mixture of heat and moisture. Leibniz does not believe in intersubstantial causation at all, so a fortiori he must reject the generation of one substance as a result of the activity of another substance.
Does this mean that Leibniz eschews spontaneity? Not entirely. As we will see in the next chapter, while Leibniz’s preformation theory compels him to reject spontaneous generation, and also compels him to see epigenetic reproduction, with Leeuwenhoek, as a sort of spontaneous generation, at the same time his theory of preestablished harmony, according to which all bodies and all souls are each, in their own way, automata, amounts to a sort of theory of universal spontaneity, according to which every state of every being unfolds according to the individual law or principle of that being. This doctrine is a well-known feature of Leibniz’s thought, yet its full significance for his philosophy of biology and of generation has yet to be treated in detail.