§ 94. _Psychology is the theory of the soul._ As we have already seen, the rise of scepticism directs attention from the object of thought to the thinker, and so emphasizes the self as a field for theoretical investigation. But the original and the dominating interest in the self is a practical one. The precept, γνῶθι σεαυτόν, has its deepest justification in the concern for the salvation of one's soul. In primitive and half-instinctive belief the self is recognized in practical relations. In its animistic phase this belief admitted of such relations with all living creatures, and extended the conception of life very generally to natural processes. Thus in the beginning the self was doubtless indistinguishable from the vital principle. In the first treatise on psychology, the "περὶ Ψυχῆς" of Aristotle, this interpretation finds a place in theoretical philosophy. For Aristotle the soul is the _entelechy_ of the body--that function or activity which makes a man of it. He recognized, furthermore, three stages in this activity: the nutritive, sensitive, and rational souls, or the vegetable, animal, and distinctively human natures, respectively. The rational soul, in its own proper activity, is man's highest prerogative, the soul to be saved. By virtue of it man rises above bodily conditions, and lays hold on the divine and eternal. But Plato, who, as we have seen, was ever ready to grant reality to the ideal apart from the circumstances of its particular embodiment, had already undertaken to demonstrate the immortality of the soul on the ground of its distinctive nature.[209:16] According to his way of thinking, the soul's essentially moral nature made it incapable of destruction through the operation of natural causes. It is evident, then, that there were already ideas in vogue capable of interpreting the Christian teaching concerning the existence of a soul, or of an inner essence of man capable of being made an object of divine interest. [Sidenote: Spiritual Substance] § 95. The immediate effect of Christianity was to introduce into philosophy as one of its cardinal doctrines the theory of a spiritual being, constituting the true self of the individual, and separable from the body. The difference recognized in Plato and Aristotle between the divine spark and the appetitive and perceptual parts of human nature was now emphasized. The former (frequently called the "spirit," to distinguish it from the lower soul) was defined as a _substance_ having the attributes of thought and will. The fundamental argument for its existence was the immediate appeal to self-consciousness; and it was further defined as indestructible on the ground of its being utterly discontinuous and incommensurable with its material environment. This theory survives at the present day in the conception of pure activity, but on the whole the attributes of the soul have superseded its substance. [Sidenote: Intellectualism and Voluntarism.]