existence is entailed by and depends on their having instances. Thus, On the Soul’s remark that universals are ‘somehow in the soul’ only reflects their role in assuring the autonomy of thought. The mind has no organ because it is not the form or first actualization of any physical structure. So, unlike perceptual faculties, it is not strongly dependent on the body. However, the mind thinks its objects by way of images, which are something like internal representations, and these are physically based. Insofar as it thus depends on imagination (phantasia), the mind is weakly dependent on the body. This would be sufficient to establish the naturalized nature of Aristotle’s mind were it not for what some consider an incurably dualist intrusion. In distinguishing something in the mind that makes all things from something that becomes all things, Aristotle introduces the notorious distinction between the active and passive intellects and may even suggest that the first is separable from the body. Opinion on the nature of the active intellect diverges widely, some even discounting it as an irrelevant insertion. But unlike perception, which depends on external objects, thinking is up to us. Therefore, it cannot simply be a matter of the mind’s being affected. So Aristotle needs a mechanism that enables us to produce thoughts autonomously. In light of this functional role, the question of active intellect’s ontological status is less pressing. Biology. Aristotle’s biological writings, which constitute about a quarter of the corpus, bring biological phenomena under the general framework of natural science: the four causes, form and matter, actuality and potentiality, and especially the teleological character of natural processes. If the Physics proceeds in an a priori style, the History of Animals, Parts of Animals, and Generation of Animals achieve an extraordinary synthesis of observation, theory, and general scientific principle. History of Animals is a comparative study of generic features of animals, including analogous parts, activities, and dispositions. Although its morphological and physiological descriptions show surprisingly little interest in teleology, Parts of Animals is squarely teleological. Animal parts, especially organs, are ultimately differentiated by function rather than morphology. The composition of, e.g., teeth and flesh is determined by their role in the overall functioning of the organism and, hence, requires teleology. Generation of Animals applies the form–matter and actuality–potentiality distinctions to animal reproduction, inheritance, and the development of accidental characteristics. The species form governs the development of an organism and determines what the organism is essentially. Although in the Metaphysics and elsewhere accidental characteristics, including inherited ones, are excluded from science, in the biological writings form has an expanded role and explains the inheritance of non-essential characteristics, such as eye color. The more fully the father’s form is imposed on the minimally formed matter of the mother, the more completely the father’s traits are passed on to the offspring. The extent to which matter resists imposition of form determines the extent to which traits of the mother emerge, or even those of more distant ancestors.
Aristotle shared the Platonists’ interest in animal classification. Recent scholarship suggests that this is less an interest in elaborating a Linnean-style taxonomy of the animal kingdom than an interest in establishing the complex differentiae and genera central to definitions of living things. The biological works argue, moreover, that no single differentia could give the whole essence of a species and that the differentiae that do give the essence will fall into more than one division. If the second point rejects the method of dichotomous division favored by Plato and the Academy, the first counters Aristotle’s own standard view that essence can be reduced to a single final differentia. The biological sciences are not, then, automatically accommodated by the Posterior Analytics model of explanation, where the essence or explanatory middle is conceived as a single causal property.
A number of themes discussed in this section are brought together in a relatively late work, Motion of Animals. Its psychophysical account of the mechanisms of animal movement stands at the juncture of physics, psychology, and biology. Metaphysics. In Andronicus’s edition, the fourteen books now known as the Metaphysics were placed after the Physics, whence comes the word ‘metaphysics’, whose literal meaning is ‘what comes after the physics’. Aristotle himself prefers ‘first philosophy’ or ‘wisdom’ (sophia). The subject is defined as the theoretical science of the causes and principles of what is most knowable. This makes metaphysics a limiting case of Aristotle’s broadly used distinction between what is better known to us and what is better known by nature. The genus animal, e.g., is better known by nature than the species man because it is further removed from the senses and because it can be known independently of the species. The first condition suggests that the most knowable objects would be the separately existing and thoroughly non-sensible objects of theology and, hence, that metaphysics is a special science. The second condition suggests that the most knowable objects are simply the most general notions that apply to things in general. This favors identifying metaphysics as the general science of being qua being. Special sciences study restricted modes of being. Physics, for instance, studies being qua having an internal principle of change and rest. A general science of being studies the principles and causes of things that are, simply insofar as they are. A good deal of the Metaphysics supports this conception of metaphysics. For example, Book IV, on the principle of non-contradiction, and Book X, on unity, similarity, and difference, treat notions that apply to anything whatever. So, too, for the discussion of form and actuality in the central books VII, VIII, and IX. Book XII, on the other hand, appears to regard metaphysics as the special science of theology. Aristotle himself attempts to reconcile these two conceptions of metaphysics. Because it studies immovable substance, theology counts as first philosophy. However, it is also general precisely because it is first, and so it will include the study of being qua being. Scholars have found this solution as perplexing as the problem. Although Book XII proves the causal necessity for motion of an eternal substance that is an unmoved mover, this establishes no conceptual connection between the forms of sensible compounds and the pure form that is the unmoved mover. Yet such a connection is required, if a single science is to encompass both. Problems of reconciliation aside, Aristotle had to face a prior difficulty concerning the very possibility of a general science of being. For the Posterior Analytics requires the existence of a genus for each science but the Metaphysics twice argues that being is not a genus. The latter claim, which Aristotle never relinquishes, is implicit in the Categories, where being falls directly into kinds, namely, the categories. Because these highest genera do not result from differentiation of a single genus, no univocal sense of being covers them. Although being is, therefore, ambiguous in as many ways as there are categories, a thread connects them. The ontological priority accorded primary substance in the Categories is made part of the very definition of non-substantial entities in the Metaphysics: to be an accident is by definition to be an accident of some substance. Thus, the different senses of being all refer to the primary kind of being, substance, in the way that exercise, diet, medicine, and climate are healthy by standing in some relation to the single thing health. The discovery of focal meaning, as this is sometimes called, introduces a new way of providing a subject matter with the internal unity required for science. Accordingly, the Metaphysics modifies the strict ‘one genus to one science’ rule of the Posterior Analytics. A single science may also include objects whose definitions are different so long as these definitions are related focally to one thing. So focal meaning makes possible the science of being qua being.
Focal meaning also makes substance the central object of investigation. The principles and causes of being in general can be illuminated by studying the principles and causes of the primary instance of being. Although the Categories distinguishes primary substances from other things that are and indicates their salient characteristics (e.g., their ability to remain one and the same while taking contrary properties), it does not explain why it is that primary substances have such characteristics. The difficult central books of the Metaphysics – VII, VIII, and IX – investigate precisely this. In effect, they ask what, primarily, about the Categories’ primary substances explains their nature. Their target, in short, is the substance of the primary substances of the Categories. As concrete empirical particulars, the latter are compounds of form and matter (the distinction is not explicit in the Categories) and so their substance must be sought among these internal structural features. Thus, Metaphysics VII considers form, matter, and the compound of form and matter, and quickly turns to form as the best candidate. In developing a conception of form that can play the required explanatory role, the notion of essence (to ti en einai) assumes center stage. The essence of a man, e.g., is the cause of certain matter constituting a man, namely, the soul. So form in the sense of essence is the primary substance of the Metaphysics. This is obviously not the primary substance of the Categories and, although the same word (eidos) is used, neither is this form the species of the Categories. The latter is treated in the Metaphysics as a kind of universal compound abstracted from particular compounds and appears to be denied substantial status.
While there is broad, though not universal, agreement that in the Metaphysics form is primary