_Ancient Theory._--The idea formerly, and almost universally entertained respecting the _modus operandi_ of the faculty we call memory, was, that in perception and the various operations of the senses, certain impressions are made on the sensorium--certain forms and types of things without, certain _images_ of them--which remain when the external object is no longer present, and become imprinted thus on the mind. Such, certainly, was the doctrine of the earliest Greek commentators on Aristotle. Such, I must think, is substantially the doctrine of Aristotle himself. _Theory of Aristotle._--His idea is, that memory, as well as imagination, primarily and directly, relates only to sensible objects, and gives us only _images_ of these objects, and even when it gives us strictly intellectual objects, gives us these only by images. One cannot _think_, he says, without images. Its source and origin, then, he concludes, is the sensibility, and so it pertains to _animals_, as well as men; only to those, however, which have the perception of time, since memory is a modification of sensation or intellectual conception, under the condition of _time past_. Such being, in his view, the nature and source of memory, he goes on to ask how it is that only a modification (or state) of the mind being present, and the object itself absent, one recalls that absent object? "Manifestly," he replies, "we must believe that the impression which is produced, in consequence of the sensation, in the soul, and in that part of the body which perceives the sensation, is analogous to a species of painting, and that the perception of that impression constitutes precisely what we call memory. The movement which then takes place in the mind _imprints there a sort of type of the sensation_ analogous to the _seal which one imprints on wax with a ring_. Hence it is that those who by the violence of the impression, or by the ardor of age are in a great excitement (movement) have not the memory of things, as if the movement and seal had been applied to running water. In the case of others, however, who are in a sort cold, as the plaster of old edifices, the very _hardness_ of the part which receives the impression prevents the image from leaving the least trace. Hence it is that young children and old men have so little memory. It is the same with those who are too lively, and those who are too slow. Neither remember well. The one class are too _humid_, the other too hard. The image dwells not in the soul of the one, makes no impression whatever on that of the other. "How is it now," he goes on to ask, "that this stamp, impression, image, or painting, in us, a mere mode of the mind, can recall the absent object?" His answer is, that the impression or image is a _copy_ of that object, while, at the same time, it is, in itself considered, only a modification of our mind, just as a painting is a mere picture, and yet a _copy_ from nature. (Parva Naturalia: Memory, ch. 1.)