CHAPTER 1. THE ARISTOTELIAN LEGACY: ANIMALS AS FORM-MATTER COMPOUNDS; REPRODUCTION AS TRANSMISSION OF FORM
It will be a central argument of this book that, overall, the place occupied by biology in Leibniz’s philosophy is closer to the one it enjoys in Aristotle than in Descartes: it is a field of application par excellence of general philosophical principles, rather than an obstacle to the viability of these principles. In this and the following chapter, we will focus on the contrast between Aristotle and Descartes, and then move on to position Leibniz in relation to them.
In this chapter, we will gain a thorough understanding of Aristotle’s approach to both general and developmental physiology, and will then move on to his account of sexual reproduction. We will conclude with an account of the central Aristotelian doctrine according to which ‘like begets like,’ that is, of his generation-based theory of species membership. Throughout, we will be concerned to position Aristotle’s account of animal motion, development, and generation in a way that will highlight its sharp differences with Descartes’s later account, in order ultimately to show the brilliant manner in which Leibniz will synthesize elements from both of these systems of biology.
In the first section, on organic structure and function in Aristotle, we will consider Aristotle’s account of the ontological priority of functions over the bodily organs through which these functions are executed, taking as our example his analysis of vision and the eye. This analysis will provide us with a first instance of the general principle that also underlies Aristotle’s account of sexual generation: that of the priority of form over matter. In Aristotle’s theory of reproduction, the father imposes both a formal and final cause, through the vehicle of the semen, upon the material of the mother’s uterus. This brings about what Jim Lennox has called “formal replication,” in which the father ensures his own eternity in kind, as Aristotle puts it, if not in number. While the product of generation is not an exact copy of the father, it is the closest thing possible to this, and the extent to which it falls short of this mark is traceable in large part to the degrading influence of the mother. The menstrual blood possesses in potentia all of the possible qualities of a human being that may be actualized once the blood is acted upon by the semen; but still the menses are not pure seed, “for it lacks one thing only, the source of the soul” (GA II 3 737a). This soul source is at the same time the source of (to speak anachronistically) the blueprint or program that determines how the fetus will develop. The semen determines the end toward which the fetus will develop, since, in Aristotle’s view, in natural beings such as animals the formal and final causes are one and the same. For Aristotle, a woman is a degradation of man, “a male deformed.” A female is generated when the movements introduced by the male’s semen prove to be weak, and are consequently overcome by the movements of the menstrual blood. Monstrosities occur, finally, when the movements of both of these are very weak and lack even the efficacy to render the menstrual blood into a distinct particular human, leaving it instead at the level of development common to all animals in general (GA IV 3). If no degradation were to occur, the male would reproduce itself perfectly. This can never happen, since particular primary substances are not eternal. The particular mechanism by which perfect self-reproduction is hindered is the mutual interaction of the movements of the semen and the menstrual blood at conception. Thus there is no need to explain where defective fetuses come from. They are merely privations of form, which manifest themselves as recapitulations, not of earlier evolutionary stages, as some modern theories would later hold, but of a more general category --animal-- to which particular members of any animal species belong.
Aristotle’s account of species membership, then, is intimately linked with his account of sexual generation: to be a member of a certain biological kind is just to have one’s form originally transmitted by one’s male progenitor, ‘for like begets like’. As we will see, this is a very convenient principle to uphold within the context of a hylomorphic metaphysics, but its truth, and even its comprehensibility, become severely problematic with the rise of mechanistic biology.
CHAPTER 2. THE CARTESIAN BACKGROUND: ANIMALS AS MACHINES; REPRODUCTION AS A THERMOMECHANICAL PROCESS
Where we limit our attention to “the manifest striking of one body against another,” to use Robert Boyle’s elegant phrase, the 17th century seems decidedly to have advanced over its Aristotelian forebears. But this focus has given us a somewhat distorted picture of the actual challenges 17th-century scientists faced. When we turn our attention to natural phenomena such as sexual generation, the early modern period looks to be in something of a crisis, as the conceptual resources the modern natural philosophers had available to them were nowhere near adequate to account for the phenomena, and indeed in many ways seemed less explanatorily adequate than what had been available in earlier centuries. The mechanist revolution was a great step forward for the understanding of rocks, planets, and cannonballs, but, in the absence of any clue about genetics, DNA, and so on, it could not but appear a great step backwards for the understanding of eggs, embryos, and animals. For this reason, it is in an important respect the non-foundational or secondary sciences such as biology, rather than physics, that may prove to be more instructive about the successes and the limitations of the scientific revolution.
In the ancient period, organic growth, driven by the internal active principle of a substance, was itself the model of natural change in general. In the 17th century, in contrast, biology begins to be displaced by mechanical physics --the study of the motion of intrinsically inert matter and of the way in which motion is imparted from one body to another-- from its role as model or paradigm science. But its displacement is not at all complete by the end of the 17th century, and precisely because organic structure, self-motion, and biological generation continued to stand as an obstacle in need of displacement, it is not at all surprising to find them at the center of the attention of all those who had any stake in the debate over how nature operates and according to what laws.
In this chapter, we will consider in detail perhaps the most radical early modern reconceptualization of vital phenomena, namely, that of René Descartes. His account of the animal as a machine, and of reproduction as a strictly thermomechanical process, would serve throughout the second half of the 17th century as the starting point for nearly all debates about the nature and origins of biological entities.
We will begin this chapter with a consideration of Descartes’s account of organic structure, with particular attention to the question as to how, and indeed whether, one can continue to account for organic functions at all within a radically deteleologized theory of the growth and self-motion of animals. We will also briefly consider Descartes’s ideas, often only implicit (and often in contradiction with his more considered views), of bodily health, sickness, and death. In the following section we will move on to a detailed study of Descartes’s Traité de la formation du foetus, as well as of connected passages from other texts, in order to gain a thorough understanding of Descartes’s attempt to account for generation and fetal development entirely in terms of what he calls ‘minor laws’. While considering his account of organogenesis, we will be addressing particular attention to his account of species reproduction and of the acquisition of particular, variable traits, for it is here that the minor laws of mechanical physics seem least able to account for the phenomena in question. Trait acquisition and species reproduction, we will see, are for Descartes the two great stumbling blocks of mechanist generation science. As Malebranche would later note, Descartes seems perfectly able to
give some explanation of the formation of the fetus in general, as [he] has tried successfully enough[. N]evertheless it is very difficult, without this communication of the mother’s brain with the child’s, to explain why a mare does not give birth to a calf, or a chicken lay an egg containing a partridge or some bird of a new species.
Leibniz as well would decry "what Monsieur des Cartes has imagined with his man, the generation of whom costs so little, but who resembles so little a true man.” For Leibniz, there is no possible way that the generation of a man or any other kind of creature could be accounted for in terms of minor laws alone. The reason for this has to do, in part, not just with the problem of the origin of the soul, but also with the particular structure of the organic body itself of the biological creature. It is to the structure, nature, and origins of biological entities in Leibniz --with a primary focus on animals (as opposed to plants and humans)-- that we will devote our attention for the rest of the book.
CHAPTER 3: THE ‘HYDRAULICO-PNEUMATICO-PYROTECHNICAL MACHINE OF QUASI-PERPETUAL MOTION’: LEIBNIZ ON ANIMAL ECONOMY
In previous chapters, we considered not just Aristotle and Descartes’s scientific treatments of the phenomena of life, but also their understanding of the place of this sort of study within their respective philosophies. It was argued that, while for Aristotle the study of the living world had been a domain of application par excellence of his most general philosophical principles, for Descartes the living world was perhaps equally important, but for an opposite reason: it was a domain to be explained away, not to be held up as paradigmatic of nature in general. What, now, was Leibniz’s understanding of the living world, and what was the place of this understanding in his philosophy as a whole?
We will be considering Leibniz’s metaphysical analysis of the animal in later chapters. In this chapter, we will focus on his rather more technical, and much less well-known, interest in the scientific study of living bodies. In particular, we will undertake a comprehensive summary and analysis of the principal unpublished physiological and medical manuscripts in the Leibniz-Handschriften III folio, from the Directiones ad rem medicam pertinentes of 1671 to the De causis et curatione febrium of 1704-5. If the predictions of the editorial staff currently at work on Series VIII of Leibniz’s Sämtliche Schriften are correct, these manuscripts will ultimately be edited and published for the Akademie Edition of Leibniz’s writings only sometime between 2020 and 2025. This book will be the first to offer a comprehensive treatment of them.
For Leibniz, the study of any element or system of the natural world --including everything from forestry to hydrology to painting-- is ultimately a part of ‘physics’. That particular subdomain of physics that deals with animal bodies is animal economy. This is a term Leibniz seems to have borrowed from Walter Charleton, who published his Oeconomia animalis novis in medicina hypothesibus superstructa et mechanice explicata in 1659. For Leibniz ‘animal economy’ denotes the study of the relation between organs and functions; thus it comprises anatomy and physiology, but particularly with an eye to the way the elements of these disciplines are coordinated in a living system. It is anatomy, physiology, and ethology at once.
What it decidedly is not is metaphysics, as Leibniz strictly eschews any appeal to the inherence of a soul in the animal body within the context of animal-economical explanation. It is not that the animal does not have a soul (indeed, as we will see in the following chapter, the animal body itself results from infinitely many souls or soul-like substances), but only that there is in his view a rigorous and autonomous scientific discipline that gains nothing from its mention. Of all of his contributions to animal economy over a forty-year period, it is only in the polemic against Stahl, the final text we will be considering in this chapter, that Leibniz seeks to make explicit the boundary between animal economy and metaphysics, in the course of his argument against Stahl that the soul can play no theoretical role in the former discipline.
To the extent that he is engaged in the study of animal economy, the animal is for Leibniz no less a machine than it had been for Descartes, but there are a number of differences between the two philosophers’ respective analyses of this machine. For one thing, Leibniz is willing to speak of animal machines in terms of their functional, as opposed to merely dispositional, unity. Machines of nature, for Leibniz, like the machines that we construct for our own reasons, have ends, and these ends may be understood in entirely mechanical terms, as rooted in the structure and motion of the mechanical body. Another difference lies in Leibniz’s ascription of species membership to animal machines, whereas, as we saw in the previous chapter, Descartes remains entirely silent on how it is that a particular animal machine is the kind of machine that it is. Yet another difference is the motivation each thinker had for studying animal economy: for Descartes, again, a complete account of the animal in mechanical terms was a necessary, if otiose, step in the comprehensive case he spent his life seeking to make for a mechanized natural philosophy. For Leibniz on the other hand the initial interest in animal economy grows out of a prior interest, not in mechanical natural philosophy, but in the advancement of the institution of medicine. Finally, whereas for Descartes the analysis of the animal qua machine constitutes an exhaustive analysis of the animal, for Leibniz it is just the beginning. In the study of animal economy, Leibniz’s model of the animal is fairly independent of the one familiar to most students of Leibniz from his broader metaphysical concern with organic bodies and corporeal substances. As we will see in the following chapters, though, once Leibniz moves beyond animal economy, and into natural philosophy and metaphysics, the animal is a good deal more than a machine.
Animal economy is, for Leibniz, the study of the animal qua machine. It is, in particular, a hydraulico-pneumatic machine, as earlier mechanists had recognized, as well as being a pyrotechnical machine, to the extent that its first motion is generated out of the production of heat in the excoction of chyle from the aliment it takes in. Because it is able to take in its own aliment, rather than being fueled by an external agent, it is a sort of perpetual-motion machine. The fact that it can eventually pass on its likeness to another machine before ceasing to exist contributes to this perpetuity, even if Leibniz acknowledges that other theorists had understood the notion of perpetual-motion machine in a rather more narrow sense.
Leibniz’s animal economy incorporates findings --most importantly from
the work of the great Danish physician Thomas Bartholin, author of the
1656 Anatomia reformata, which Leibniz studied and on which he wrote
extensive comments-- from a domain-specific science of animal
chemistry, such as those concerning the excoction of chyle, or the
fermentation that gives rise to vital heat. It is however distinct
from chemistry, to the extent that chemistry takes the animal as a
mass, while animal economy is intent to understand it as a structure.
Animal economy is, so to speak, doubly economical, both in that the
designer of natural machines brings about the maximum of effects with
the minimum of organs, and with individual organs fulfilling multiple
functions; and in the sense that it eschews in principle any appeal to
the inherence of a soul in the animal for the explanation of vital
phenomena. The Leibnizian animal is, unlike that of Descartes, an
ensouled entity, but the soul is responsible only for perception and
appetition, functions that are beyond the scope of the study of animal
economy.
CHAPTER 4. ORGANIC BODY AND CORPOREAL SUBSTANCE
We have seen that, in the study of animal economy, Leibniz is no less a
mechanist than Descartes. He does have an original view of the animal
machine, as a hydraulico-pneumatico-pyrotechnic machine of
quasi-perpetual motion, whereas Descartes would only have recognized
the first half of this definition. But Leibniz agrees nonetheless with
Descartes that for the purposes of the science of animal economy the
living being may be studied exhaustively as a ‘mere’ machine. There is
a deeper level, however, at which the animal body is not a ‘mere’
machine but a special kind of machine, a ‘more exquisite [exquisitior]’
or ‘more divine [divinior]’ machine: this is the machine of nature or
the organic body, whose exquisiteness resides in the fact that it
remains a machine in its least parts, which is to say that there is no
stage in its decomposition at which one arrives at non-machinic
components.
This more exquisite mechanism may be seen as an intermediary step between the mere machine studied by animal economy on the one hand, and on the other the corporeal substance of interest to the metaphysician. If in its broad outlines the machine of Leibniz’s animal economy comes from the Cartesian tradition, while the corporeal substance of his metaphysics is fundamentally Aristotelian, the organic body we consider in the present chapter is entirely Leibniz’s own invention. It has no historical precedents, or at least none in philosophy (though some scholars, such as Antonio Nunziante, have discerned clear literary precedents, including Cyrano de Bergerac's Les Empires de la Lune). Leibniz does not give a name to the project of analyzing the structure and nature of organic bodies, but we might call it ‘organics’, on analogy to the science of mechanics.
Neither in animal economy nor in organics does the soul need to be invoked in order to understand the living entity under investigation. At the level of the organic body, however, Leibniz has left behind the traditional mechanists who view the living entity as a mere machine, and replaced this model with, as he puts it, a machine that remains a machine in its least parts. The principle purpose of this chapter is to make sense of this new sort of machine. A good deal hangs, philosophically, on understanding the machine of nature, since, as we will have learned in the previous chapter, in his polemic against Stahl Leibniz hopes to be able to derive the vegetative force or vital activity of living beings from this structure alone.
As we will see, Leibniz’s full account of how this derivation might be carried through emerges out of his debate with the Cambridge Platonists over the need to invoke immaterial plastic natures for the explanation of growth, development and motion in living nature, and it draws on the deepest principles of both his metaphysics—in particular the doctrine of preestablished harmony—and of his dynamics—in particular the concept of derivative force. In coming to understand Leibniz’s model of the organic body, then, we will also learn why he believes that plastic natures and other immaterial principles of growth and development may be dispensed with, and why he holds that an updated mechanism is adequate for the scientific explanation of all natural phenomena, biological as well as physical.
As I hope to go some way towards showing in this chapter, organic bodies are not incompatible with the simple substances at the heart of Leibniz’s supposed immaterialism. Indeed, organic bodies flow directly, as modifications, from the primitive active and passive forces of these simple substances. We are thus entitled, without entering into conflict with the immaterialist interpretation of Leibniz’s deepest metaphysical convictions, to attribute to Leibniz the view that substances in fact have real, physical bodies, which are themselves composed out of other substances with real, physical bodies, and so on to infinity. Corporeal substances consist in part in physical bodies, and so do all of the infinitely many substances implicated in these bodies. In seeking to determine what considerations led Leibniz to propose this picture of substance, some of the first and most visible road-signs appear to point in the direction of the empirical life sciences. The connection between these sciences and Leibniz’s metaphysics will be the topic of the following chapter.
CHAPTER 5. LEIBNIZ’S MODEL OF THE ORGANIC BODY IN THE LIGHT OF EMPIRICAL LIFE SCIENCE
In this chapter, we will be focusing on the relation of Leibniz’s model of organic body to the empirical life sciences of Leibniz’s era, and in particular to the 17th century’s discovery of the ubiquity of subvisible living creatures, as well as to the era's growing awareness of the deep interdependence of all living entities. It will be argued that this interdependence was an important influence on Leibniz’s model of organic body, and on his conception of corporeal substances as consisting in infinite unions of what might be called nested individuals.
This chapter will begin with some admittedly speculative considerations, in direct response to earlier speculation on the part of commentators such as Jacques Roger and Catherine Wilson, as to Leibniz’s motivation for taking an interest in the natural world, and as to whether we are at all justified in attempting to link this interest to his ‘deeper’ philosophical convictions. We will then move on to a detailed analysis of Leibniz’s model of the individual organism as one consisting in infinitely many nested individuals, and we will show that this model is directly connected to shifting ideas in Leibniz’s era as to the inherent interdependence of biological organisms. It will be shown that Leibniz’s model is indicative of a shift from the implicit principle of Aristotelian physiology, ‘one body, one substance,’ to a general acceptance of what might be called ‘universal symbiosis’. On the basis of a detailed historical account of Leibniz’s exposure to microscopical research throughout his life, from his first mention of Athanasius Kircher’s discoveries in a 1669 letter to Thomasius to his repeated mentions in the Monadology of the research of Swammerdam, Leeuwenhoek, and Malpighi, I shall attempt to textually ground the claim as to the relevance of microscopy to Leibniz’s metaphysics.
The chapter will then conclude with a consideration of the legacy of Leibniz’s interest in biology in 18th-century natural science and philosophy. As we will see, for the first chapter of his long and storied reception history, Leibniz was understood by naturalist thinkers such as Bonnet, Maupertuis, and Tremblay, as principally a philosopher of biology, who elaborated a metaphysical system on the basis of what he observed in the natural world, rather than on the basis of abstract a priori principles. The version of Leibniz current in French natural science will not be defended here as the correct one, in contrast to the much better known logician and metaphysician promoted by Leibniz's German successors such as Wolff, Baumgarten, and Kant, and then canonized in the early 20th century as a result of the influential interpretations of Leibniz by Bertrand Russell and Louis Couturat. Rather, the naturalist Leibniz celebrated in 18th-century French natural science will be held up as an example of how extremely variable are the uses to which this versatile philosopher may be put.
CHAPTER 6. SPERMATOZOA, PREESTABLISHED HARMONY, AND THE IMMORTALITY OF CORPOREAL SUBSTANCE
Leibniz inherits his belief in the immortality of all souls from a long, broadly speaking Platonic, philosophical tradition. He also believes, in view of his commitment to the impossibility of causal influence between souls and bodies, that “la nature soit parallèle a la grace.” This parallelism requires that the preexisting soul preexist “dans une manière de corps organisé.” Thus, there is a preestablished harmony between bodies and souls, between the kingdoms of nature and of grace. What’s more, given the perpetual union of souls and bodies, and given the fact that organic bodies have the infinite structure they do, it must be the case that all organic bodies originated concomitantly with souls. Not only have all souls existed since the beginning of time, but so have all corporeal substances. “This doctrine,” Leibniz argues, “is well confirmed by the microscopic observations of M. Leeuwenhoek and other fine observers” (Théodicée § 91).
Leeuwenhoek discovers spermatozoa in the mid-1670s, when Leibniz’s theory of preexisting corporeal substances is still taking shape. Eventually, Leibniz will come to identify the spermatozoa discovered by Leeuwenhoek as the bodies of preexisting souls. Bodies have to be found for the souls after death as well, since a disembodied soul would be a soul removed from its preestablished harmony with the physical world. Again, Leibniz looks to natural science --this time to the insect metamorphosis studied by Jan Swammerdam-- and finds evidence that living things never really lose their bodies, but only transform to the point of unrecognizability.
Johannes Bertram maintains in a polemical work of 1741 that Leibniz “would never have come to the fantastic concepts of his monads and little spirits” had it not been for the discovery of spermatozoa.” This is of course entirely untrue, though it is also of tremendous interest as a corroboration of the general tendency among Leibniz’s interpreters in the 18th century, described in the previous chapter, to emphasize the “disjunct” of Leibniz’s metaphysics (in Robinet’s expression) that takes substance as a corporeal, active entity such as an animal, rather than as an absolutely simple node of perception. Where the spermatozoa do come in handy for Leibniz is as an empirical corroboration of the particular subdoctrine of the theory of monads according to which no monad may exist entirely apart from the world of bodies. If every true unity must have always existed, and could never have existed in a disembodied form—both of which principles gain Leibniz’s assent due to ‘higher’ or non-empirical reasons—then obviously the discovery of a preexisting bodily vehicle for a corporeal substance, such as an animal or a human that is destined eventually to enter into the ‘larger theater’, is a very useful one indeed.
CHAPTER 7. ORGANIC DESIGN, ORGANIC CHANGE, AND THE ONTOLOGY OF BIOLOGICAL SPECIES
The problem of species has long been familiar to analytic philosophers interested in Leibniz. Particularly in the treatment Leibniz offers of the problem in his Nouveaux essais sur l’entendement humain of 1704 --a comprehensive response to Locke’s Essay Concerning Human Understanding of 1690-- it has been understood principally as a problem in metaphysics and the philosophy of language: how it is, namely, that the meanings we give to words can be determined to capture the real natures of the things in the world that these words are purported to denote. What has been largely overlooked is that for both Leibniz and Locke the determination of the boundaries of species, if there are any, is also a problem in the philosophy of biology.
In this chapter, we will see that Leibniz’s analysis of the nature and boundaries of biological species is very different from his now well-known nominalist account of the species of, e.g., mathematical objects, and even of ordinary physical objects. With respect to plants and animals, Leibniz positions himself squarely in the species-fixist camp, like his contemporary the English naturalist John Ray, who insists unequivocally that “the number of true species in nature is fixed and limited and, as we may reasonably believe, constant and unchangeable from the first creation to the present day.” Leibniz believes that all species were formed at the Creation and remain fixed for all time, notwithstanding his simultaneous belief in the possibility of tremendous morphological change in a species over time, and even of tremendous morphological change or ‘transformation’ over the course of an individual creature’s life. Whatever his views with respect to other ontological domains, such as that of mathematical objects, being a biological species fixist Leibniz is eo ipso a species realist, not a nominalist, and this notwithstanding his commitment to the principle of plenitude or to an infinite hierarchy of beings. After all, in order for there to be a biological species between any two given species, there must be such a thing as species. These are not species, moreover, in the sense in which Thomas Aquinas had understood the species of angels; for Leibniz, a biological species is multiply instantiated, and to be a member of the same species as another creature is precisely to share in the same line of descent from shared ancestors as that other creature.
A biological species, then, is for Leibniz a permanently isolated reproductive community, all of whose members may be said to be members of the same species not in virtue of any morphological resemblances (though they generally have these as well), but rather in virtue of shared origins. This account of species, while starkly different from Aristotle’s (treated in chapter 1), had come by the 16th century to be associated with Aristotle’s biology, but in fact emerges out of the combined impact of the exigencies of the new science of taxonomy, particularly botanical taxonomy, and a new literalism in scriptural interpretation. Leibniz is, then, very much of his time, a contemporary of John Ray and a biological thinker squarely in the line of development that extends from Cesalpino in the 16th century to Linnaeus in the 18th.
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