The difference in philosophic method between Plato and Aristotle is well illustrated by their treatment of memory. What Plato says of memory is incidental to the discussion of such profound matters as the nature of the soul and the theory of knowledge. Memory, according to him, is one of the higher faculties and partakes of the eternal nature of the soul. Aristotle makes a special study of memory in a less transcendental way. With him it is no longer a function of the eternal _Nous_; but it has its seat in the passive Reason, is dependent upon a physiological process, and perishes with the body. Aristotle seems to have been the first of ancient philosophers to write a systematic treatise on psychology. But, rather curiously, in this work on psychology there is no special treatment of memory. A special tract, however, was devoted to the subject.[9] This, so far as we know, was the first scientific study of memory; and for this reason, as well as for its intrinsic merits, the tract deserves special attention. But before passing to his doctrine of memory, it is well to notice briefly his theory of sense-perceptions. On occasion of appropriate stimuli movements take place in the sense-organs. These movements, however, are not sense perception. In perception the mind must compare and distinguish disparate sensations; it must unite the sensations presented simultaneously by our double sense organs as of sight and hearing, and it must be conscious of sensation. This work of comparison, of psychic synthesis, and of self-conscious perception is performed by a central sense. The physical basis of this sense is the heart. Through it the mind performs the act of sense perception. The functions now attributed to nervous substance are referred by Aristotle to the _pneuma_ connected with the blood. This is the medium by which the movements arising in the sense organs are transmitted to the heart, and in this _pneuma_ the movements persist long after the external stimuli have ceased to act. Incidentally, it is interesting to note, that according to Aristotle’s psychology, the brain has very little to do with mental activity. To borrow a phrase from Wallace, it serves simply as “a cooling apparatus to counteract the excessive warmth of the heart.” When the movement occasioned in the sense organ by an external stimulus is propagated to the heart,[10] it becomes a perception of the soul. Sense perception, then, is an act of the soul by means of a physiological process. In the words of Aristotle it is “a movement of the soul through the body.”[11] Now this movement sometimes continues after the stimulus, which was the occasion of it, has ceased to act. The extreme case is the well known phenomenon of a visual after-image. The images of the imagination are such after-sensations. Imagination is weak sensation, or in the words of Hobbes, “decaying sense”. So too, dreaming is the result of a movement in our bodily organs, caused either from without or from within.