In his Posterior Analytics Aristotle applies the theory of the syllogism to scientific and epistemological ends. Scientific knowledge, he urges, must be built up out of demonstrations. A demonstration is a particular kind of syllogism, one whose premises can be traced back to principles that are true, necessary, universal, and immediately intuited. These first, self-evident principles are related to the conclusions of science as axioms are related to theorems: the axioms both necessitate and explain the truths that constitute a science. The most important axioms, Aristotle thought, would be those that define the proper subject matter of a science (thus, among the axioms of geometry would be the definition of a triangle). For this reason much of the second book of the Posterior Analytics is devoted to definition.
The account of science in the Posterior Analytics is impressive, but it bears no resemblance to any of Aristotle’s own scientific works. Generations of scholars have tried in vain to find in his writings a single instance of a demonstrative syllogism. Moreover, the whole history of scientific endeavour contains no perfect instance of a demonstrative science.
Philosophy of mind
Aristotle regarded psychology as a part of natural philosophy, and he wrote much about the philosophy of mind. This material appears in his ethical writings, in a systematic treatise on the nature of the soul (De anima), and in a number of minor monographs on topics such as sense-perception, memory, sleep, and dreams.
For Aristotle the biologist, the soul is not—as it was in some of Plato’s writings—an exile from a better world ill-housed in a base body. The soul’s very essence is defined by its relationship to an organic structure. Not only humans but beasts and plants too have souls, intrinsic principles of animal and vegetable life. A soul, Aristotle says, is “the actuality of a body that has life,” where life means the capacity for self-sustenance, growth, and reproduction. If one regards a living substance as a composite of matter and form, then the soul is the form of a natural—or, as Aristotle sometimes says, organic—body. An organic body is a body that has organs—that is to say, parts that have specific functions, such as the mouths of mammals and the roots of trees.
The souls of living beings are ordered by Aristotle in a hierarchy. Plants have a vegetative or nutritive soul, which consists of the powers of growth, nutrition, and reproduction. Animals have, in addition, the powers of perception and locomotion—they possess a sensitive soul, and every animal has at least one sense-faculty, touch being the most universal. Whatever can feel at all can feel pleasure; hence, animals, which have senses, also have desires. Humans, in addition, have the power of reason and thought (logismos kai dianoia), which may be called a rational soul. The way in which Aristotle structured the soul and its faculties influenced not only philosophy but also science for nearly two millennia.
Aristotle’s theoretical concept of soul differs from that of Plato before him and René Descartes (1596–1650) after him. A soul, for him, is not an interior immaterial agent acting on a body. Soul and body are no more distinct from each other than the impress of a seal is distinct from the wax on which it is impressed. The parts of the soul, moreover, are faculties, which are distinguished from each other by their operations and their objects. The power of growth is distinct from the power of sensation because growing and feeling are two different activities, and the sense of sight differs from the sense of hearing not because eyes are different from ears but because colours are different from sounds.
The objects of sense come in two kinds: those that are proper to particular senses, such as colour, sound, taste, and smell, and those that are perceptible by more than one sense, such as motion, number, shape, and size. One can tell, for example, whether something is moving either by watching it or by feeling it, and so motion is a “common sensible.” Although there is no special organ for detecting common sensibles, there is a faculty that Aristotle calls a “central sense.” When one encounters a horse, for example, one may see, hear, feel, and smell it; it is the central sense that unifies these sensations into perceptions of a single object (though the knowledge that this object is a horse is, for Aristotle, a function of intellect rather than sense).
Besides the five senses and the central sense, Aristotle recognizes other faculties that later came to be grouped together as the “inner senses,” notably imagination and memory. Even at the purely philosophical level, however, Aristotle’s accounts of the inner senses are unrewarding.
At the same level within the hierarchy as the senses, which are cognitive faculties, there is also an affective faculty, which is the locus of spontaneous feeling. This is a part of the soul that is basically irrational but is capable of being controlled by reason. It is the locus of desire and passion; when brought under the sway of reason, it is the seat of the moral virtues, such as courage and temperance. The highest level of the soul is occupied by mind or reason, the locus of thought and understanding. Thought differs from sense-perception and is the prerogative, on earth, of human beings. Thought, like sensation, is a matter of making judgments; but sensation concerns particulars, while intellectual knowledge is of universals. Reasoning may be practical or theoretical, and, accordingly, Aristotle distinguishes between a deliberative and a speculative faculty.
In a notoriously difficult passage of De anima, Aristotle introduces a further distinction between two kinds of mind: one passive, which can “become all things,” and one active, which can “make all things.” The active mind, he says, is “separable, impassible, and unmixed.” In antiquity and the Middle Ages, this passage was the subject of sharply different interpretations. Some—particularly among Arab commentators—identified the separable active agent with God or with some other superhuman intelligence. Others—particularly among Latin commentators—took Aristotle to be identifying two different faculties within the human mind: an active intellect, which formed concepts, and a passive intellect, which was a storehouse of ideas and beliefs.
If the second interpretation is correct, then Aristotle is here recognizing a part of the human soul that is separable from the body and immortal. Here and elsewhere there is detectable in Aristotle, in addition to his standard biological notion of the soul, a residue of a Platonic vision according to which the intellect is a distinct entity separable from the body. No one has produced a wholly satisfactory reconciliation between the biological and the transcendent strains in Aristotle’s thought.