of fire (M) belongs to the clouds (S) and (1H) Noise (P) belongs to quenching of fire (M). Because what is explained in science is invariant and holds of necessity, the premises of a scientific or demonstrative syllogism must be necessary. In requiring that the premises be prior to and more knowable than the conclusion, Aristotle embraces the view that explanation is asymmetrical: knowledge of the conclusion depends on knowledge of each premise, but each premise can be known independently of the conclusion. The premises must also give the causes of the conclusion. To inquire why P belongs to S is, in effect, to seek the middle term that gives the cause. Finally, the premises must be immediate and non-demonstrable. A premise is immediate just in case there is no middle term connecting its subject and predicate terms. Were P to belong to M because of a new middle, M1, then there would be a new, more basic premise, that is essential to the full explanation. Ultimately, explanation of a received fact will consist in a chain of syllogisms terminating in primary premises that are immediate. These serve as axioms that define the science in question because they reflect the essential nature of the fact to be explained – as in (1H) the essence of thunder lies in the quenching of fire. Because they are immediate, primary premises are not capable of syllogistic demonstration, yet they must be known if syllogisms containing them are to constitute knowledge of the conclusion. Moreover, were it necessary to know the primary premises syllogistically, demonstration would proceed infinitely or in a circle. The first alternative defeats the very possibility of explanation and the second undermines its asymmetric character. Thus, the primary premises must be known by the direct grasp of the mind (noûs). This just signals the appropriate way for the highest principles of a science to be known – even demonstrable propositions can be known directly, but they are explained only when located within the structure of the relevant science, i.e., only when demonstrated syllogistically. Although all sciences exhibit the same formal structure and use certain common principles, different sciences have different primary premises and, hence, different subject matters. This ‘one genus to one science’ rule legislates that each science and its explanations be autonomous. Aristotle recognizes three kinds of intellectual discipline. Productive disciplines, such as house building, concern the making of something external to the agent. Practical disciplines, such as ethics, concern the doing of something not separate from the agent, namely, action and choice. Theoretical disciplines are concerned with truth for its own sake. As such, they alone are sciences in the special sense of the Posterior Analytics. The three main kinds of special science are individuated by their objects – natural science by objects that are separate but not changeless, mathematics by objects that are changeless but not separate, and theology by separate and changeless objects. The mathematician studies the same objects as the natural scientist but in a quite different way. He takes an actual object, e.g. a chalk figure used in demonstration, and abstracts from or ‘thinks away’ those of its properties, such as definiteness of size and imperfection of shape, that are irrelevant to its standing as a perfect exemplar of the purely mathematical properties under investigation. Mathematicians simply treat this abstracted circle, which is not separate from matter, as if it were separate. In this way the theorems they prove about the object can be taken as universal and necessary.
Physics. As the science of nature (physis), physics studies those things whose principles and causes of change and rest are internal. Aristotle’s central treatise on nature, the Physics, analyzes the most general features of natural phenomena: cause, change, time, place, infinity, and continuity. The doctrine of the four causes is especially important in Aristotle’s work. A cause (aitia) is something like an explanatory factor. The material cause of a house, for instance, is the matter (hyle) from which it is built; the moving or efficient cause is the builder, more exactly, the form in the builder’s soul; the formal cause is its plan or form (eidos); and the final cause is its purpose or end (telos): provision of shelter. The complete explanation of the coming to be of a house will factor in all of these causes. In natural phenomena efficient, formal, and final causes often coincide. The form transmitted by the father is both the efficient cause and the form of the child, and the latter is glossed in terms of the child’s end or complete development. This explains why Aristotle often simply contrasts matter and form. Although its objects are compounds of both, physics gives priority to the study of natural form. This accords with the Posterior Analytics’ insistence that explanation proceed through causes that give the essence and reflects Aristotle’s commitment to teleology. A natural process counts essentially as the development of, say, an oak or a man because its very identity depends on the complete form realized at its end. As with all things natural, the end is an internal governing principle of the process rather than an external goal. All natural things are subject to change (kinesis). Defined as the actualization of the potential qua potential, a change is not an ontologically basic item. There is no category for changes. Rather, they are reductively explained in terms of more basic things – substances, properties, and potentialities. A pale man, e.g., has the potentiality to be or become tanned. If this potentiality is utterly unactualized, no change will ensue; if completely actualized, the change will have ended. So the potentiality must be actualized but not, so to speak, exhausted; i.e., it must be actualized qua potentiality. Designed for the ongoing operations of the natural world, the Physics’ definition of change does not cover the generation and corruption of substantial items themselves. This sort of change, which involves matter and elemental change, receives extensive treatment in On Generation and Corruption. Aristotle rejects the atomists’ contention that the world consists of an infinite totality of indivisible atoms in various arrangements. Rather, his basic stuff is uniform elemental matter, any part of which is divisible into smaller such parts. Because nothing that is actually infinite can exist, it is only in principle that matter is always further dividable. So while countenancing the potential infinite, Aristotle squarely denies the actual infinite. This holds for the motions of the sublunary elemental bodies (earth, air, fire, and water) as well as for the circular motions of the heavenly bodies (composed of a fifth element, aether, whose natural motion is circular). These are discussed in On the Heavens. The four sublunary elements are further discussed in Meteorology, the fourth book of which might be described as an early treatise on chemical combination. Psychology. Because the soul (psyche) is officially defined as the form of a body with the potentiality for life, psychology is a subfield of natural science. In effect, Aristotle applies the apparatus of form and matter to the traditional Greek view of the soul as the principle and cause of life. Although even the nutritive and reproductive powers of plants are effects of the soul, most of his attention is focused on topics that are psychological in the modern sense. On the Soul gives a general account of the nature and number of the soul’s principal cognitive faculties. Subsequent works, chiefly those collected as the Parva naturalia, apply the general theory to a broad range of psychological phenomena from memory and recollection to dreaming, sleeping, and waking.
The soul is a complex of faculties. Faculties, at least those distinctive of persons, are capacities for cognitively grasping objects. Sight grasps colors, smell odors, hearing sounds, and the mind grasps universals. An organism’s form is the particular organization of its material parts that enable it to exercise these characteristic functions. Because an infant, e.g., has the capacity to do geometry, Aristotle distinguishes two varieties of capacity or potentiality (dynamis) and actuality (entelecheia). The infant is a geometer only in potentiality. This first potentiality comes to him simply by belonging to the appropriate species, i.e., by coming into the world endowed with the potential to develop into a competent geometer. By actualizing, through experience and training, this first potentiality, he acquires a first actualization. This actualization is also a second potentiality, since it renders him a competent geometer able to exercise his knowledge at will. The exercise itself is a second actualization and amounts to active contemplation of a particular item of knowledge, e.g. the Pythagorean theorem. So the soul is further defined as the first actualization of a complex natural body.
Faculties, like sciences, are individuated by their objects. Objects of perception (aisthesis) fall into three general kinds. Special (proper) sensibles, such as colors and sounds, are directly perceived by one and only one sense and are immune to error. They demarcate the five special senses: sight, hearing, smell, taste, and touch. Common sensibles, such as movement and shape, are directly perceived by more than one special sense. Both special and common sensibles are proper objects of perception because they have a direct causal effect on the perceptual system. By contrast, the son of Diares is an incidental sensible because he is perceived not directly but as a consequence of directly perceiving something else that happens to be the son of Diares – e.g., a white thing.
Aristotle calls the mind (noûs) the place of forms because it is able to grasp objects apart from matter. These objects are nothing like Plato’s separately existing Forms. As Aristotelian universals, their