It is thus that Plato begins his Psychology with Kosmology: the Kosmos is in his view a divine immortal being or animal, composed of a spherical rotatory body and a rational soul, cognitive as well as motive. Among the tenants of this Kosmos are included, not only gods, who dwell in the peripheral or celestial regions, but also men, birds, quadrupeds, and fishes. These four inhabit the more central or lower regions of air, earth, and water. In describing men and the inferior animals, Plato takes his departure from the divine Kosmos, and proceeds downwards by successive stages of increasing degeneracy and corruption. The cranium of man was constructed as a little Kosmos, including in itself an immortal rational soul, composed of the same materials, though diluted and adulterated, as the kosmical soul; and moving with the like rotations, though disturbed and irregular, suited to a rational soul. This cranium, for wise purposes which Plato indicates, was elevated by the gods upon a tall body, with attached limbs for motion in different directions — forward, backward, upward, downward, to the right and left.2 Within this body were included two inferior and mortal souls: one in the thoracic region near the heart, the other lower down, below the diaphragm, in the abdominal region; but both of them fastened or rooted in the spinal marrow or cord, which formed a continuous line with the brain above. These two souls were both emotional; the higher or thoracic soul being the seat of courage, energy, anger, &c., while to the lower or abdominal soul belonged appetite, desires, love of gain, &c. Both of them were intended as companions and adjuncts, yet in the relation of dependence and obedience, to the rational soul in the cranium above; which, though unavoidably debased and perturbed by such unworthy companionship, was protected partially against the contagion by the difference of location, the neck being built up as an isthmus of separation between the two. The thoracic soul, the seat of courage, was placed nearer to the head, in order that it might be the medium for transmitting influence from the cranial soul above, to the abdominal soul below; which last was at once the least worthy and the most difficult to control. The heart, being the initial point of the veins, received the orders and inspirations of the cranial soul, transmitting them onward through its many blood-channels to all the sensitive parts of the body; which were thus rendered obedient, as far as possible, to the authority of man’s rational nature.3 The unity or communication of the three souls was kept up through the continuity of the cerebro-spinal column.

But, though by these arrangements the higher soul in the cranium was enabled to control to a certain extent its inferior allies, it was itself much disturbed and contaminated by their reaction. The violence of passion and appetite, the constant processes of nutrition and sensation pervading the whole body, the multifarious movements of the limbs and trunk, in all varieties of direction, — these causes all contributed to agitate and to confuse the rotations of the cranial soul, perverting the arithmetical proportions and harmony belonging to them. The circles of Same and Diverse were made to convey false information; and the soul, for some time after its first junction with the body, became destitute of intelligence.4 In mature life, indeed, the violence of the disturbing causes abates, and the man may become more and more intelligent, especially if placed under appropriate training and education. But in many cases no such improvement took place, and the rational soul of man was irrecoverably spoiled; so that new and worse breeds were formed, by successive steps of degeneracy. The first stage, and the least amount of degeneracy, was exhibited in the formation of woman; the original type of man not having included diversity of sex. By farther steps of degradation, in different ways, the inferior animals were formed — birds, quadrupeds, and fishes.5 In each of these, the rational soul became weaker and worse; its circular rotations ceased with the disappearance of the spherical cranium, and animal appetites with sensational agitations were left without control. As man, with his two emotional souls and body joined on to the rational soul and cranium, was a debased copy of the perfect rational soul and spherical body of the divine Kosmos, so the other inhabitants of the Kosmos proceeded from still farther debasement and disrationalization of the original type of man.

Such is the view of Psychology given by Plato in the Timæus; beginning with the divine Kosmos, and passing downwards from thence to the triple soul of man, as well as to the various still lower successors of degenerated man. It is to be remarked that Plato, though he puts soul as prior to body in dignity and power, and as having for its functions to control and move body, yet always conceives soul as attached to body, and never as altogether detached, not even in the divine Kosmos. The soul, in Plato’s view, is self-moving and self-moved: it is both Primum Mobile in itself, and Primum Movens as to the body; it has itself the corporeal properties of being extended and moved, and it has body implicated with it besides.

The theory above described, in so far as it attributes to the soul rational constituent elements (Idem, Diversum), continuous magnitude, and circular rotations, was peculiar to Plato, and is criticized by Aristotle as the peculiarity of his master.6 But several other philosophers agreed with Plato in considering self-motion, together with motive causality and faculties perceptive and cognitive, to be essential characteristics of soul. Alkmæon declared the soul to be in perpetual motion, like all the celestial bodies; hence it was also immortal, as they were.7 Herakleitus described it as the subtlest of elements, and as perpetually fluent; hence it was enabled to know other things, all of which were in flux and change. Diogenes of Apollonia affirmed that the element constituent of soul was air, at once mobile, all-penetrating, and intelligent. Demokritus declared that among the infinite diversity of atoms those of spherical figure were the constituents both of the element fire and of the soul: the spherical atoms were by reason of their figure the most apt and rapid in moving; it was their nature never to be at rest, and they imparted motion to everything else.8 Anaxagoras affirmed soul to be radically and essentially distinct from every thing else, but to be the great primary source of motion, and to be endued with cognitive power, though at the same time not suffering impressions from without.9 Empedokles considered soul to be a compound of the four elements — fire, water, air, earth; with love and hatred as principles of motion, the former producing aggregation of elements, the latter, disgregation: by means of each element the soul became cognizant of the like element in the Kosmos. Some Pythagoreans looked upon the soul as an aggregate of particles of extreme subtlety, which pervaded the air and were in perpetual agitation. Other Pythagoreans, however, declared it to be an harmonious or proportional mixture of contrary elements and qualities; hence its universality of cognition, extending to all.10

A peculiar theory was delivered by Xenokrates (who, having been fellow-pupil with Aristotle under Plato, afterwards conducted the Platonic School, during all the time that Aristotle taught at the Lykeium), which Aristotle declares to involve greater difficulty than any of the others. Xenokrates described the soul as “a number (a monad or indivisible unit) moving itself.”11 He retained the self-moving property which Plato had declared to be characteristic of the soul, while he departed from Plato’s doctrine of a soul with continuous extension. He thus fell back upon the Pythagorean idea of number as the fundamental essence. Aristotle impugns, as alike untenable, both the two properties here alleged — number and self-motion. If the monad both moves and is moved (he argues), it cannot be indivisible; if it be moved, it must have position, or must be a point; but the motion of a point is a line, without any of that variety that constitutes life. How can the soul be a monad? or, if it be, what difference can exist between one soul and another, since monads cannot differ from each other except in position? How comes it that some bodies have souls and others not? and how, upon this theory, can we explain the fact that many animated bodies, both plants and animals, will remain alive after being divided, the monadic soul thus exhibiting itself as many and diverse? Besides, the monad set up by Xenokrates is hardly distinguishable from the highly attenuated body or spherical atom recognized by Demokritus as the origin or beginning of bodily motion.12

These and other arguments are employed by Aristotle to refute the theory of Xenokrates. In fact, he rejects all the theories then current. After having dismissed the self-motor doctrine, he proceeds to impugn the views of those who declared the soul to be a compound of all the four elements, in order that they might account for its percipient and cognitive faculties upon the maxim then very generally admitted13 — That like is perceived and known by like. This theory, the principal champion of which was Empedokles, appears to Aristotle inadmissible. You say (he remarks) that like knows like; how does this consist with your other doctrine, that like cannot act upon, or suffer from, like, especially as you consider that both in perception and in cognition the percipient and cognizant suffers or is acted upon?14 Various parts of the cognizant subject, such as bone, hair, ligaments, &c., are destitute of perception and cognition; how then can we know anything about bone, hair, and ligaments, since we cannot know them by like?15 Suppose the soul to be compounded of all the four elements; this may explain how it comes to know the four elements, themselves, but not how it comes to know all the combinations of the four; now innumerable combinations of the four are comprised among the cognita. We must assume that the soul contains in itself not merely the four elements, but also the laws or definite proportions wherein they can combine; and this is affirmed by no one.16 Moreover, Ens is an equivocal, or at least a multivocal, term; there are Entia belonging to each of the ten Categories. Now the soul cannot include in itself all the ten, for the different Categories have no elements in common; in whichever Category you rank the soul, it will know (by virtue of likeness) the cognita belonging to that category, but it will not know the cognita belonging to the other nine.17 Besides, even if we grant that the soul includes all the four elements, where is the cementing principle that combines all the four into one? The elements are merely matter; and what holds them together must be the really potent principle of soul; but of this no explanation is given.18

Some philosophers have assumed (continues Aristotle) that soul pervades the whole Kosmos and its elements; and that it is inhaled by animals in respiration along with the air.19 They forget that all plants, and even some animals, live without respiring at all; moreover, upon this theory, air and fire also, as possessing soul, and what is said to be a better soul, ought (if the phrase were permitted) to be regarded as animals. The soul of air or fire must be homogeneous in its parts; the souls of animals are not homogeneous, but involve several distinct parts or functions.20 The soul perceives, cogitates, opines, feels, desires, repudiates; farther, it moves the body locally, and brings about the growth and decay of the body. Here we have a new mystery:21 — Is the whole soul engaged in the performance of each of these functions, or has it a separate part exclusively consecrated to each? If so, how many are the parts? Some philosophers (Plato among them) declare the soul to be divided, and that one part cogitates and cognizes, while another part desires. But upon that supposition what is it that holds these different parts together? Certainly not the body (which is Plato’s theory); on the contrary, it is the soul that holds together the body; for, as soon as the soul is gone, the body rots and disappears.22 If there be anything that keeps together the divers parts of the soul as one, that something must be the true and fundamental soul; and we ought not to speak of the soul as having parts, but as essentially one and indivisible, with several distinct faculties. Again, if we are to admit parts of the soul, does each part hold together a special part of the body, as the entire soul holds together the entire body? This seems impossible; for what part of the body can the Noûs or Intellect (e.g.) be imagined to hold together? And, besides, several kinds of plants and of animals may be divided, yet so that each of the separate parts shall still continue to live; hence it is plain that the soul in each separate part is complete and homogeneous.23

Aristotle thus rejects all the theories proposed by antecedent philosophers, but more especially the two following:—That the soul derives its cognitive powers from the fact of being compounded of the four elements; That the soul is self-moved. He pronounces it incorrect to say that the soul is moved at all.24 He farther observes that none of the philosophers have kept in view either the full meaning or all the varieties of soul; and that none of these defective theories suffices for the purpose that every good and sufficient theory ought to serve, viz., not merely to define the essence of the soul, but also to define it in such a manner that the concomitant functions and affections of the soul shall all be deducible from it.25 Lastly, he points out that most of his predecessors had considered that the prominent characteristics of soul were — to be motive and to be percipient:26 while, in his opinion, neither of these two characteristics is universal or fundamental.

Aristotle requires that a good theory of the soul shall explain alike the lowest vegetable soul, and the highest functions of the human or divine soul. And, in commenting on those theorists who declared that the essence of soul consisted in movement, he remarks that their theory fails altogether in regard to the Noûs (or cogitative and intellective faculty of the human soul); the operation of which bears far greater analogy to rest or suspension of movement than to movement itself.27

We shall now proceed to state how Aristotle steers clear (or at least believes himself to steer clear) of the defects that he has pointed out in the psychological theories of his predecessors. Instead of going back (like Empedokles, Plato, and others) to a time when the Kosmos did not yet exist, and giving us an hypothesis to explain how its parts came together or were put together, he takes the facts and objects of the Kosmos as they stand, and distributes them according to distinctive marks alike obvious, fundamental, and pervading; after which he seeks a mode of explanation in the principles of his own First Philosophy or Ontology. Whoever had studied the Organon and the Physica of Aristotle (apparently intended to be read prior to the treatise De Animâ) would be familiar with his distribution of Entia into ten Categories, of which Essence or Substance was the first and the fundamental. Of these Essences or Substances the most complete and recognized were physical or natural bodies; and among such bodies one of the most striking distinctions, was between those that had life and those that had it not. By life, Aristotle means keeping up the processes of nutrition, growth, and decay.28

“To live” (Aristotle observes) is a term used in several different meanings; whatever possesses any one of the following four properties is said to live:29 (1) Intellect, (2) Sensible perception, (3) Local movement and rest, (4) Internal movement of nutrition, growth, and decay. But of these four the last is the only one common to all living bodies without exception; it is the foundation presupposed by the other three. It is the only one possessed by plants,30 and common to all plants as well as to all animals — to all animated bodies.

What is the animating principle belonging to each of these bodies, and what is the most general definition of it? Such is the problem that Aristotle states to himself about the soul.31 He explains it by a metaphysical distinction first introduced (apparently) by himself into Philosophia Prima. He considers Substance or Essence as an ideal compound; not simply as clothed with all the accidents described in the nine last Categories, but also as being analysable in itself, even apart from these accidents, into two abstract, logical, or notional elements or principia — Form and Matter. This distinction is borrowed from the most familiar facts of the sensible world — the shape of solid objects. When we see or feel a cube of wax, we distinguish the cubic shape from the waxen material;32 we may find the like shape in many other materials — wood, stone, &c.; we may find the like material in many different shapes — sphere, pyramid, &c.; but the matter has always some shape, and the shape has always some matter. We can name and reason about the matter, without attending to the shape, or distinguishing whether it be cube or sphere; we can name and reason about the shape, without attending to the material shaped, or to any of its various peculiarities. But this, though highly useful, is a mere abstraction or notional distinction. There can be no real separation between the two: no shape without some solid material; no solid material without some shape. The two are correlates; each of them implying the other, and neither of them admitting of being realized or actualized without the other.

This distinction of Form and Matter is one of the capital features of Aristotle’s Philosophia Prima. He expands it and diversifies it in a thousand ways, often with subtleties very difficult to follow; but the fundamental import of it is seldom lost — two correlates inseparably implicated in fact and reality in every concrete individual that has received a substantive name, yet logically separable and capable of being named and considered apart from each other. The Aristotelian analysis thus brings out, in regard to each individual substance (or Hoc Aliquid, to use his phrase), a triple point of view: (1) The Form; (2) The Matter; (3) The compound or aggregate of the two — in other words, the inseparable Ens, which carries us out of the domain of logic or abstraction into that of the concrete or reality.33

Aristotle farther recognizes, between these two logical correlates, a marked difference of rank. The Form stands first, the Matter second, — not in time, but in notional presentation. The Form is higher, grander, prior in dignity and esteem, more Ens, or more nearly approaching to perfect entity; the Matter is lower, meaner, posterior in dignity, farther removed from that perfection. The conception of wax, plaster, wood, &c., without amy definite or determinate shape, is confused and unimpressive; but a name, connoting some definite shape, at once removes this confusion, and carries with it mental pre-eminence, alike as to phantasy, memory, and science. In the logical hierarchy of Aristotle, Matter is the inferior and Form the superior;34 yet neither of the two can escape from its relative character: Form requires Matter for its correlate, and is nothing in itself or apart,35 just as much as Matter requires Form; though from the inferior dignity of Matter we find it more frequently described as the second or correlate, while Form is made to stand forward as the relatum. For complete reality, we want the concrete individual having the implication of both; while, in regard to each of the constituents per se, no separate real existence can be affirmed, but only a nominal or logical separation.

This difference of rank between Matter and Form — that the first is inferior and the last the superior — is sometimes so much put in the foreground, that the two are conceived in a different manner and under other names, as Potential and Actual. Matter is the potential, imperfect, inchoate, which the supervening Form actualizes into the perfect and complete; a transition from half-reality to entire reality or act. The Potential is the undefined or indeterminate36 — what may be or may not be — what is not yet actual, and may perhaps never become so, but is prepared to pass into actuality when the energizing principle comes to aid. In this way of putting the antithesis, the Potential is not so much implicated with the Actual as merged and suppressed to make room for the Actual: it is as a half-grown passing into a full-grown; being itself essential as a preliminary stage in the order of logical generation.37 The three logical divisions — Matter, Form, and the resulting Compound or Concrete (τὸ σύνολον, τὸ συνειλημμένον), are here compressed into two — the Potential and the Actualization thereof. Actuality (ἐνέργεια, ἐντελέχεια) coincides in meaning partly with the Form, partly with the resulting Compound; the Form being so much exalted, that the distinction between the two is almost effaced.38