The doctrine of transmigration as held in the Pythagoreans is an anticipation of Plato’s doctrine of reminiscence, but there is little psychology in it. Theophrastus tells us that Diogenes of Appollonia was puzzled by the phenomenon of forgetting things.[2] But he explained it in accordance with the principles of his philosophy by supposing that the cause of forgetting was an arrest of the equal distribution of air throughout the body. A corroboration of this explanation he found in the easier breathing that follows the recalling of what was forgotten. Among the Eleatics, Parmenides is reported to have held that not only thought, but recollecting and forgetting depended upon the way the light or heat and the dark or cold are mixed in the body. If we may trust Theophrastus,[3] it may be assumed that, according to Parmenides, every presentation corresponded to a definite mixture or relation of these qualities, and with the destruction of that relation the presentation disappeared, that is, was forgotten. Heraclitus, one might suppose, would study memory carefully, but in the fragments of his philosophy that have come down to us nothing is said upon the subject. In Plato we find a more modern psychology. According to him the thinking power of the mind, the understanding, is above the mere power of sense perceptions. It is this power which compares and considers, notes similarities and contrasts, unity and plurality, and forms ideas of relation between Being and Non-Being as well as relations of number and proportions. Among the elements of this power, recollection (ἀνάμνησις) is of prime importance. This rests upon the association by similarity and simultaneity.[4] Plato distinguishes the passive retention (μνήμη) of perceptions from active memory (ἀνάμ@ν@ησις),[5] and suggests as a definition of memory, “the power which the soul has of recovering, when by itself, some feeling which it experienced when in company with the body.” He attempts no explanation of memory ¬of memory¬; but in the Theaetetus puts the following words into the mouth of Socrates: “I would have you imagine, then, that there exists in the mind of man a block of wax, which is of different sizes in different men; harder, moister, and having more or less of purity in one than in another, and in some of an intermediate quality.... Let us say that this tablet is a gift of Memory, the mother of the Muses, and that when we wish to remember anything which we have seen or heard, or thought in our own minds, we hold the wax to the perceptions and thoughts, and in that receive the impressions from them as from the seal of a ring; and that we remember and know what is imprinted as long as the image lasts; but when the image is effaced, or cannot be taken, then we forget and do not know.”