And again

"To what part of the soul, then, does memory belong? The answer is clear: to the same part as the imagination. The essential objects of memory [i.e. sensory perceptions] are the fabric of the imagination, and without imagination its accidental objects [i.e. thoughts] would not occur.

"Someone may be puzzled: if affection is present to the mind, and the real-life thing is excluded, then how is anything remembered in its absence? For it is clear that what is thus produced through sense perception in the soul and its affiliated region in the body must be thought of as some kind of still-life [zōgráphēma], and the fact of having it we call memory: as it occurs, the movement leaves something like an imprint of the perception as its sign, as people do when using signet-rings to make a seal."

"But if this truly describes the events of memory, which is it that we remember: the affection, or that thing from which it was produced? If it is the former, we would be unable to remember anything in its absence. And if it is the latter, given that what we sense is the affection, [the same question arises:] how do we remember the absent thing which we do not presently sense? And if something similar to an imprint or an inscription is inside us, how is our perception of it equal to the memory of something else, and not the selfsame thing? For when memory is actualized, what one contemplates and perceives is this affection.

"So how will one remember a thing when it is not there? One might as well be able to see and hear what is not there. Is this what in fact happens, and how it is received? For even as a picture painted on a panel is both a picture and a portrayal, and being one and the same thing is nevertheless endowed with different sorts of being, and it is possible to regard it both as a picture [unto itself] and a likeness [of something else] -- even so, we must apprehend our interior phántasma as an image with being unto itself, as well as being the image of something else. Insofar as it is unto itself, it is an object of contemplation and a mental image. Inso far as it is of something else, it is a likeness and a memory."

On Memory and Recollection 450a22-32, 450b11-27

Aristotle on the same

"On the subject of perception in its totality, we must understand it as what receives the forms of things sensed in isolation from their matter, the way wax receives the device [sēmeîon] of a signet-ring in isolation from its iron or silver: the iron or silver device is taken in, but not the iron or silver as such. Similarly, the senses are in every case affected by a thing's color, taste or noise -- not that which a thing is said to be, but qualities which it has and are accounted for."

"If thinking is indeed comparable to perception, it is either an affection caused by what is thinkable or something of the kind. Though thought itself is unaffected, it must be receptive of its object's form, and able to be like it while remaining something else. As the sensory faculty is to the object of sense, so must mind be to the thinkable. Now since the mind thinks all things, in order to exercise its force -- that is, to know -- it must as Anaxagoras says be 'unmixed' (59B12 DK), for any foreign object that comes in from outside hinders and obstructs it. Thus the mind can have no nature but a potential one. So that part of the soul called 'mind' (with which the soul does its thinking and conceiving) has no actual existence until it thinks. For this reason it cannot rightly be said to mix with the body either. For then it would take on qualities like hot or cold, or it would reside in an organ as do the senses. But it has none. It is not wrong to call the soul a place of forms, as long as this is restricted to its thinking capacity, and the forms are understood to be potential and not actual.

"The senses and the mind are dissimilar in their imperviousness to affection. On considering the sensory organs and sensation this is easy to see, for after an intense sensation the sense is rendered inoperative -- as with the sense of hearing after a loud sound, or the inability to see or smell in the wake of strong colors and aromas. But the mind that thinks an intense thought finds that lesser thoughts are heightened, not diminished."

On the Soul II.12, III.4

The well-known simile

SOCRATES: Do my account the favor of supposing that our souls contain a slab of wax. This slab varies in size, purity and hardness from individual to individual, and in some cases its consistency is ideal.
THEAETETUS: So I shall suppose.
SOCRATES: Let us go on to say it is a gift from Mnemosyne, mother of the Muses, and that it works like this: of all we see or hear or think of on our own, we lay the slab beneath those thoughts and perceptions we wish to remember and create an impression, just like the seals we stamp with signet-rings. Whatever is imprinted, we hold in our memory and our knowledge as long as its image holds, but what is washed away or is impossible to imprint becomes forgotten and unknown.
     ....Knowing you and Theodorus, I retain in that waxiness signs for both of you, as if left by signet-rings. Now when from far off I get an indistinct view of you together, I strive to tally a particular sign against a particular sight, and to make it fit the trace of its own imprint. But in my rush I fumble my attempt at recognition, and match the sight of one to the other's sign, as when people bind their sandals to the wrong feet. You could also compare it to what happens in a mirror, when right and left trade places and confound the sight. By this type of error, judgment is parted from the truth and rendered false.
THEAETETUS: A very likely account, Socrates, and a marvelous description of what can happen to our judgment.
SOCRATES: Besides this there occurs the case where, knowing you both, I perceive one of you but not the other, and my knowledge of the other is not in accordance with my perception of him. When I described it like this previously you did not understand me.
THEAETETUS: I sure didn't.
SOCRATES: Earlier [at 192b], I was saying that one man who knows another and perceives him and holds his knowledge of him in accordance with his perception would never confuse him with a third man he knows and perceives, as long as his knowledge of the third man was also held in accordance with his perception. Was that not it?
THEAETETUS: Yes.
SOCRATES: Now I'm talking about about what went missing from that account. It's the case in which we say false opinion arises: when, knowing two men and seeing them both (or perceiving them by some other means), we fail to hold the sign of each in accordance with the perception, and like a poor archer straying from the target we get it wrong. And this is what we call falsity.
THEAETETUS: Quite fairly.
SOCRATES: So any time a sign is met by its corresponding perception but another sign corresponding to an absent perception gets fastened to it, thought is deceived. By this account (if what we are saying is correct), it is impossible to be deceived or judge falsely about things unknown and never perceived. Rather, it is about the things we know and perceive that judgment turns and darts in its alternation between falsity and truth: true when it aligns the proper stamps precisely with what made them, and false when it gets them crooked and aslant.

Theaetetus 191c-e, 193c-194b

Philolaus of Croton

“All things which are known have number. For it is impossible that anything at all be recognized or known, without this.”

"Number has two types all its own, odd and even... And of both types there are many forms, which each particular thing indicates [autò sēmaínei]."

44B4-5 (DK)

A story of the painter Apelles

They say that when he was painting a horse, he became so frustrated in his wish to depict the horse's lather that he flung the sponge he used for wiping pigments off his brushes—and that its impact on the picture replicated the lather of a horse.

Sextus Empiricus, Outlines of Pyrrhonism 1.28