The first point is to understand what is to be implied in the term “soul.” Plato was really a dualist in that he regards the soul as a separate entity which animates the body and compares it to a rider directing and controlling the horse he rides. But Aristotle makes a more careful analysis of psychological phenomena. In the treatise _de anima_ he says “there is no need to enquire whether soul and body are one, any more than whether the wax and the imprint are one; or, in general, whether the matter of a thing is the same with that of which it is the matter.” (Aristot: _de anima_. II. i. 412. b. 6.) Aristotle defines the soul as “the first actuality of a natural body having in it the capacity of life” (id. 412. b. 5), in which “first” denotes that the soul is the primary form by which the substance of the body is actualized, and “actuality” refers to the actualizing principle by which form is given to the body which otherwise would be only a collection of separate parts each having its own form but the aggregate being without corporate unity until the soul gives it form; in this sense the soul is the realization of the body (cf. Aristot: _Metaph._ iii. 1043. a. 35). A dead body lacks this actualizing and centralizing force and is only a collection of limbs and organs, yet even so it is not an artificial collection such as a man might put together, but “a natural body having in it the capacity of life,” that is to say, an organic structure designed for a soul which is the cause or reason of its existence and which alone enables the body to realize its object.
The soul contains four different faculties or powers which are not strictly to be taken as “parts” though in the passage cited above Aristotle uses the term “parts.” These are, (i) the nutritive, the power of life whereby the body performs such functions as absorbing nourishment, propagating its species, and other functions common to all living beings, whether animal or vegetable: (ii) the sensible, by which the body obtains knowledge through the medium of the special senses of sight, hearing, touch, etc., and also the “common sense” by means of which these perceptions are combined, compared, and contrasted so that general ideas are obtained which ultimately rest on the sense perceptions: (iii) the locomotive, which prompts to action, as desire, appetite, will, etc., also based, though indirectly, on sense perception, being suggested by memories of senses already in action: (iv) the intellect or pure reason, which is concerned with abstract thought and is not based on sense perception. All these, embracing life in its widest application, are classed together as soul, but the last, the intellect, _nous_, or rational soul, is peculiar to man alone. It does not depend on the senses, directly or indirectly, and so, whilst the other three faculties necessarily cease to function when the bodily organs of sense cease, it does not necessarily follow that this rational soul will cease as it is apparently independent of the organ sense. This _nous_ or “spirit” is reduced by Aristotle to a much more restricted range than is usual in the older philosophers and is taken to mean that which has the capacity of abstract knowledge, independent of the information due, directly or indirectly, to sense perception. It would seem, however, to be a distinct species of faculty for Aristotle says: “As regards intellect and the speculative faculty the case is not yet clear. It would seem, however, to be a distinct species of soul, and it alone is capable of separation from the body, as that which is eternal from that which is perishable. The remaining parts of the soul are, as the foregoing consideration shows, not separable in the way that some allege them to be: at the same time it is clear that they are logically distinct.” (Arist. _de anima._ II. ii. 413. b. 9). It is suggested that (i) the rational soul is of a distinct species and so presumably derived from a different source than the other faculties of the soul, but nothing is said as to whence it is derived: (ii) it is capable of existence independently of the body, that is to say its activity does not depend on the operation of the bodily organs, but it is not stated that it does so exist; (iii) it is eternal on the ground that it can exist apart from the perishable.