Again, these persisting movements are the elements of memory. At first, one wonders how Aristotle will distinguish those movements which constitute memory from those which are the basis of imagination. He is not entirely satisfactory on this point; but he makes the following distinction. The picture of the imagination, or the corresponding movement does not refer to an external object, and is not located in the past. The memory picture, on the other hand, does refer to an object and carries with it the consciousness of a time in the past, when the perception remembered took place.[12] Memory, then, involves time, and both this and the sense of time are _dependent_ upon the central sense. Memory, as we have seen, is dependent upon the residua of sensations. The subjective side of a sensation is an image. Thus memory belongs to the same part of the soul as the imagination, and the proper objects @of memory@ are images (φανταστὰ). The image is a condition (πάθος) of the central sense. Memory _per se_ is of the original image or perception, and only in an accidental manner does it relate to matters of thought.[13] In his special tract on memory, Aristotle in part repeats Plato’s views, in part discusses the obvious facts of memory, which, having been continually repeated since his time, have become mere platitudes, and in part he tries to explain the phenomena of memory in accordance with his general system of psychology. The essay, however, is of special interest, because in it Aristotle sets forth very clearly the famous doctrine of association of ideas. Some of the other points in the treatise may be briefly mentioned and special consideration given to the portion relating to recollection and association. ¶ First Aristotle takes considerable space to show what would seem to be apparent enough to everybody, that memory is of the past, as perception is of the present, and hope and opinion of the future. The central sense or sensorium must be in a condition suitable to receive and retain impressions. If the sensorium is too hard, no impression is made. If it is greatly agitated, the new movement is ineffectual: on somewhat the same principle, one may suppose, as one say in modern psychology, that a weak stimulus is washed out by a strong one. Hence the very young and the very old have poor memories; for the former are in the movement of growth, the latter in that of decay. Again, the question arises: How is it that in recollection we recognize the memory image as a picture of the absent object? A scholastic answer is given. “An animal painted in a picture, he says, is both an animal and a copy, and while being thus one and the same, it is nevertheless two things at once. The animal and the copy are not identical, and we may think of the picture either as animal or as a representation. This is also true of the image within us; and the idea which the mind contemplates is something, although it is also the image of something else.”[14]