53. First Group of Problems: The Activities of the Soul. -- All the phenomena of life are found united in man. Aristotle attaches them sometimes to four, sometimes to five faculties, basing his division on the irreducible forms of vital activity: nutrition, sensation, locomotion, understanding, and sometimes appetition.

(1) Nutrition, whose psychological aspect was carefully studied by Aristotle, is in point of view of finality the primordial vital function, since it preserves the life of the living thing. Nutrition is an assimilation of the unlike to the like; it requires heat, which is supplied by the heart. The pneuma is the air which we breathe and which gets warm by contact with the organic heat. The functions of generation are akin to those of nutrition.

(2) Sense-knowledge. -- Plato had neglected this domain, and his predecessors had reduced the function to a mere mechanical action of like upon like. Aristotle's theory is a new and masterly one, in conformity with undeniable facts of observation.

There are different forms of Sense-knowledge. Aristotle distinguished the five external senses; the common sense (aisthêtêrion koinon), which is a central organ that takes cognisance of the action of the external senses and associates our special sensations with one another; memory (mnêmê, anamnêsis), or the imagination, which retains and reproduces the impressions made on the senses; and the constructive imagination (phantasia).

The most important of Aristotle's theories concerns the nature of actual sensation (aisthêsis). The sense faculty represents sensible, particular, contingent properties; these constitute either the proper object of the scope or activity of some one particular sense (the "proper sensibles"), or an object which this sense perceives in common with other senses (the "common sensibles"). The union of the knowing with the known belongs to the psychical, not the physical, order; the sentient subject and sense-perceived object are one in the act of sensation, for this latter is the common act of both. The nature of sensation will be best understood by taking account of its genesis.

Genesis of Sensation. The senses do not act of themselves; they need to be stimulated and internally determined by some external object, which will thus become the term of the perceptive act. When the motion caused from without (the "sensible in potency") strikes the passive faculty within, this latter passes into act (the "sensible in act"), and this immanent act is sense-knowledge. The living image imprinted by the object on the organs of sense becomes a known image. This double phase of knowledge -- the action of the external object on the faculty, and the reaction of the latter -- takes place within us and is of the psychical order.

Aristotle engrafted on this Philosophical theory a scientific theory, that of the milieu or medium. The external object, he says, does not act directly on the organs of sense, but only through some medium, air and water for sight, hearing, smell, and taste; the flesh for the sense of touch (the sensations of touch not taking place on the surface of the body, but having their seat in the heart). Whether this direct influence on the faculty of knowledge comes from the object itself or from a physical medium, the psychological difficulty remains the same. In either case, a material agent contributes to the production of a psychical phenomenon, the nature of which is unexplained.

The determinant of the psychical phenomenon, or the action of the obect received in the faculty, was called later on, the intentional species (species intentionalis). Now, the Greek commentators of Aristotle misinterpreted his theory of the species intentionalis. After the manner of Democritus (10), and under the pretext that this transmission of influence through a medium should preserve during its whole course a likeness with the object, they imagined a veritable generation by the external object in the physical medium, of a succession of small, shadowy entities, the last of which became incorporated with the faculty previous to the act of knowledge. It is important to note that this false interpretation, which has played such an important part in the history of philosophy, has nothing in common with the real thought of the Stagirite.

Sensation is objectively valid. As the senses are determined only by the influence of an object, it follows that the latter must have a real existence outside us. Furthermore, it must resemble the forms of knowledge which it produces in our faculties.

(3) Intellectual Knowledge. -- Intelligence (nous) does not belong to animal nature, it is proper to man. We must first consider the nature of thought. Whilst the senses know only the concrete, particular, contingent object, the intellect perceives the "quiddity" (or "essence") of the sensible thing, apart from its individual characteristics and its limitation in time and space. It discerns the reality under abstract, and therefore also universal and immutable, aspects. The theory of abstraction, the keystone in the arch of peripatetic ideology, accounts for the distinctive properties of thought, whilst avoiding the errors of Plato's "dialectic".

Next as regards the genesis of thought. We have no innate ideas (Plato). The understanding is a capacity for knowing everything. Indeterminate and passive in its nature, like the sense-faculties, it resembles a "virgin sheet of paper" (Treatise on the Soul, iii., 4, i). This is the passive intellect. As soon as this intellect receives the determining action of the object of cognition it reacts, and by reacting knows. But whence comes this action? What is its source, -- seeing that the abstract object of the intellect does not exist as such in nature? (43). It results from two causes: the sense-image which is a necessary antecedent and concomitant of all thought; and an active faculty which co-operates with the sense-image -- in which the intelligible object is contained "potentially," -- and renders this image capable of determining the intellect. Besides the passive intellect which is the "receptacle of the determining forms" and which "can become everything," there is an active intellect which "produces everything" (Treatise on the Soul, iii., 5, 1). This is yet another application of the theory of potency and act. To make use of a well-known comparison, the active intellect "illuminates" the sense-image just as light renders colours visible and "makes the medium actually transparent". The concept of a psychical determinant reappears here, built on the pure notion of passive potency.

What is the nature of this Twofold Intellect? On the strength of the principle that what acts is superior to what is acted upon (op. cit., iii., 5, 2), Aristotle establishes between these two intellects fundamental points of difference. The active intellect alone is independent of the body, having existed before it, and surviving dissolution; it comes from without (thurathen). On the other hand, the passive intellect is akin to sensibility, is born with the organism and dies with it. The active intellect, being a "divine" principle, is impassible; it is always in act but is never acted upon; it has no power of memory and can give us no information about its state of pre-existence.

This theory of the two intellects is very obscure in many points, and full of difficulties: Is the passive intellect, -- and, as a corollary, is thought itself, -- material or spiritual? Is the active intellect one for all men, or is it part of the soul? Can it have knowledge by itself apart, or, since knowledge is a mode or quality of the passive intellect, does it not find itself condemned to absolute inaction on its separation from the body? How are we to explain its union with the passive intellect, and how does this union harmonize with the personality and unity of the individual man? What are the relations between the active intellect and the Pure Act? These questions will be seen to provoke in the course of later history very diverse and conflicting answers.

What of the objectivity of thought? Thought reproduces reality, faithfully but not adequately. The quiddity, which the intellect grasps in the sense-image, constitutes the thing known; but the abstract and universal form in which thought grasps it, is the product of the intellect itself (43).

(4) Appetition follows and depends on knowledge; it is a tendency of the being towards a known object which presents the character of goodness. Besides the sense-appetite, there is an intellectual one: the will (55). Liberty is a result of the will's autonomy; it entails responsibility.

(5) Knowledge and appetite direct the executive faculties, -- of locomotion or change in space.