Among the early Christian philosophers we find a mixture of Greek and Christian ideas. The characteristics of this period show the tre mendous hold which the spiritual ideas of Chris tianity had taken on the strongest minds. The writings of the Apologists, the Church Fathers, particularly Clement of Alexandria and Origen, while they do not reveal any systematic doctrine of the soul, are replete nevertheless with the keenest insight. The profound analysis of Augus tine, however, made positive contributions to the problem. Anticipating Descartes, he maintained that it is impossible for thought to be an at tribute of that which does not think; even if I doubt, the doubt itself must be an act of the soul and therefore a real fact of spiritual sig nificance. If the soul were corporeal, its func tions would be limited to the perception of body; but now it has the power of reflection, of knowl edge, of love, and is, above all, conscious of itself, and therefore cannot be an attribute of extended substance merely (De Trinitate). The theories developed under Scholasticism are for the most part adaptations of the later Greek ideas to the necessities of Church doctrine and authority. Hence we find some inclining to take the view of Plato that the finite soul is part of a world soul, as that idea was developed in Stoicism and later Jewish Hellenism; others incline to Aris totle's teleological conception of the soul as a cause realizing itself in the different grades of reality.
It was Descartes who brought reflection back from the region of scholastic metaphysics to the subjective side of the problem. Descartes dis covered, as Augustine and William of Auvergne had discovered before him, that to doubt the existence of the soul is to contradict one's self; for doubt is a mental fact, and as such has reality. I that doubt, think; I may imagine that I have no body, but as long as I think I have real existence; I think, there fore I am (cogito ergo sum). If it. he replied that my thinking does not imply reality then the reply is: God cannot deceive us. and His omnipotence can realize everything we conceive; therefore every clear and distinct idea we have must he real, and since I have a clear and dis tinct idea of myself and of my body in their distinction, it follows that soul and body are dis tinct and may exist without each other. Thought and extension are two attributes, and it is thought alone which it is impossible for us to doubt. Thus body and soul are left opposed to each other, so far, at any rate, as man is con cerned. Spinoza sought to obtain a unity of the two (thought and extension) and formulated the conception of an underlying soul-substance which, as God, differentiates itself in infinite and eter nal modes or attributes which are characterized under the categories of thought and extension.
Thus body and soul are ultimate realities of one and the same substance, the ever-changing coun terparts of each other, and yet the modes of one infinite reality. Leibnitz, not satisfied with the pantheism of Spinoza, sought, in his theory of atoms or monads, to retain the rights of finite personality and things and yet to avoid the crude dualism of Descartes. All things have souls according to Leibnitz: the world consists of an infinite number of them, in all degrees of perfection. If we ask for the nature of their life, inner experience reveals to us an active, real force, namely, our souls, and this is the type of all substance; so that in the world both kinds of reality, thought and extension, consist of perceiving soul-life. With this view may be compared that of Berkeley, who carried idealism to its extreme expression in his dictum that the being of things is in their being perceived (esse pereipi)• The Empiricists, Hobbes, Locke, Hume, and Mill, developed their views of the soul along the lines laid down by Bacon. Hobbes is openly materialistic: but he is offset by the cautious psychology of Locke, who finds that inner feel ing undoubtedly gives us the consciousness of self, though not the substance which underlies it, which is an unknown quantity whose real ex istence we can neither dogmatically affirm nor deny. These ideas Hume carried to their logical conclusion by denying any existence to the soul as a real or permanent subject: the only reality we know is the phenomenal stream of impres sions and ideas. It was the merit of this analy sis of Hume that it finally woke up Kant, whose views have greatly influenced recent thought. By an analysis of the human reason Kant sought to show that the real significance of the soul con sists in the moral or practical activity, which an accurate knowledge of the laws of thought could do nothing successful to overthrow. If the sys tem of Kant caused a theoretical schism be tween the reason as the knowing activity and the will as the moral activity, the reflections of Fichte, Schelling, and Hegel, together with the labors of the modern school of psychology, have done much to heal the breach. On the whole, therefore, we may say that the hypothesis of a soul seems to be demanded both as a ground of the unity of self-consciousness and also of the universe. It seems, moreover, to be justified. with sufficient reason, as the real principle of the harmony of the subjective and the objective. It seems also to be required as the subject of the changing states of thought. feeling, and voli tion, revealed in the phenomena of conscious ness.