Plato accounts for memory in the “Theaetetus” by the image of the seal ring which impresses wax; the character and duration of the impression depends upon the size, purity, and hardness of the wax. Fichte says, “The spirit does not conserve its products,--the single ideas, volitions, and feelings are conserved by the mind and constitute the ground of its inexhaustibly retentive memory.... The possibility of recalling what has once been independently done, this remains in the spirit.” James Sully compares the receptivity of memory with the infusion of dampness into an old MS. Draper also brings a physical example: If you put a flat object upon the surface of a cold, smooth metal and then breathe on the metal and, after the moisture has disappeared, remove the object, you may recall its image months after, whenever you breathe on the place in question. Another has called memory the safe of the mind. It is the opinion of E. Hering[214] that what we once were conscious of and are conscious of again, does not endure as image but as echo such as may be heard in a tuning fork when it is properly struck. Reid asserts that memory does not have present ideas, but past things for its object. Natorp explains recollection as an identification of the unidentical, of not-now with now. According to Herbart and his school,[215] memory consists in the possibility of recognizing the molecular arrangements which had been left by past impressions in the ganglion cells, and in reading them in identical fashion. According to Wundt and his pupils, the problem is one of the disposition of the central organs. And it is the opinion of James Mill that the content of recollection is not only the idea of the remembered object, but also the idea that the object had been experienced before. Both ideas together constitute the whole of that state of mind which we denote as memory. Spinoza[216] deals freely with memory, and asserts that mankind does not control it inasmuch as all thoughts, ideas, resolutions of spirits, are bare results of memories, so that human freedom is excluded. Uphues[217] distinguishes between memory and the conception which is presupposed in the recognition of an object different from that conception. This is the theory developed by Aristotle.