Book II, Chapter 5

Now that these matters have been determined, let us speak in general terms about sense-perception as a whole. Perception, as has been said, comes about in being moved and being acted upon; for it seems to be a kind of alteration. And some say, further, that like is acted upon by like. In what way this is possible or impossible we have stated in our general discussions of acting and being acted upon.

But there is a difficulty as to why there is no perception of the senses themselves, and why they produce no perception without external objects, although fire and earth and the other elements are present in them, of which, either in themselves or in their accidents, there is perception. It is clear, then, that the perceptive faculty is not in actuality but only in potentiality. Hence it is like the combustible, which is not burnt of itself, by itself alone, without something to set it burning; for otherwise it would burn itself, and would have no need of fire existing in actuality.

Since we speak of perceiving in two ways (for we say of what hears and sees potentially that it hears and sees, even if it happens to be asleep, and equally of what is already actively doing so), perception too would be spoken of in two ways, the one as in potentiality, the other as in actuality. And likewise perceiving, both that which is in potentiality and that which is in actuality.

First, then, let us speak as though being acted upon, being moved, and being active were one and the same; for movement too is a kind of actuality, though an incomplete one, as has been said elsewhere. But everything is acted upon and moved by an agent which is in actuality. Hence in one way a thing is acted upon by what is like it, and in another way by what is unlike it, as we said; for it is while unlike that a thing is acted upon, but when it has been acted upon it is like.

We must also draw distinctions concerning potentiality and actuality (entelecheia); for at present we are speaking of them without qualification. For there is a sense in which a thing is a knower, as we should call a man a knower because man belongs to the class of knowers and of those that possess knowledge; but there is another sense, in which we call a knower the man who already possesses grammar. Each of these two has his capacity in a different way: the one because his genus and his matter are of such a kind, the other because he is able, whenever he wishes, to contemplate, provided nothing external prevents him; whereas the man who is already contemplating is in actuality and in the strict sense knows this particular A. Both the former, then, are knowers potentially; but the one becomes a knower by being altered through learning, and by repeatedly changing over from a contrary condition, while the other passes in another way, from possessing sense-perception or grammar without exercising it, to the exercise of it.

Nor is "being acted upon" a simple notion either: in one sense it is a kind of destruction by the contrary, in another it is rather the preservation of what is in potentiality by what is in actuality and like it, in the way that a potentiality stands to an actuality. For that which possesses knowledge comes to be contemplating, and this is either not an alteration at all (since the development is into itself and into actuality) or else a different kind of alteration. Hence it is not right to say that the thinking subject is altered when it thinks, any more than the builder is altered when he builds.

That, then, which leads what thinks and understands from potentiality to actuality deserves in justice to be called not teaching but by some other name. But of that which, starting from potentiality, learns and acquires knowledge under the agency of what is in actuality and able to teach, we must either say, as has been said, that it is not acted upon at all, or else that there are two modes of alteration, the change to privative dispositions and the change to positive states and to a thing's nature.

In the case of the perceptive faculty, the first change is brought about by the parent; and once the offspring has been born, it already possesses perceiving in the way in which it possesses knowledge. And actual perceiving is spoken of in the same way as contemplating; but it differs in this, that in perception the things which produce the actuality are external — the visible and the audible, and likewise the rest of the perceptible objects. The reason is that actual perception is of particulars, whereas knowledge is of universals; and these are somehow in the soul itself. Hence it lies with a man himself to think whenever he wishes, but to perceive does not lie with him; for the perceptible object must be present. And the same holds also of the sciences which deal with perceptible things, and for the same reason, namely that perceptible things are among particulars and among external things.

But there will be another occasion, later on, to make these matters clear. For the present let this much be determined: that what is called potential is not a single thing, but is said in one sense as we should say that a boy is capable of being a general, and in another as we say it of a man of the right age; and it is in this latter way with the perceptive faculty. Since the difference between them has no name, though it has been determined about them that they are different and how they are different, we must use "being acted upon" and "being altered" as though they were the proper terms. The perceptive faculty is potentially such as the perceptible object already is in actuality, as has been said. It is acted upon, then, while unlike; but when it has been acted upon it has been made like, and is such as that object is.

Book II, Chapter 6

We must now speak of each sense in turn, and first of the objects of perception. The term "perceptible object" is used in three ways: two of these we say are perceived in themselves, while one is perceived incidentally. Of the two, the one is proper to each single sense, the other common to them all.

By "proper" I mean that which cannot be perceived by another sense, and about which it is impossible to be deceived: for instance, sight is of colour, hearing of sound, and taste of flavour. Touch, indeed, has more than one kind of difference. But each sense at any rate discriminates about these, and is not deceived that a thing is a colour or that it is a sound, though it may be deceived about what the coloured thing is or where it is, or what the sounding thing is or where.

Such things, then, are called the objects proper to each sense; while the common objects are movement, rest, number, figure, magnitude. For things of this kind are proper to no one sense, but common to them all; for a certain movement is perceptible both by touch and by sight.

A thing is said to be perceptible incidentally when, for example, the white thing should happen to be the son of Diares; for it is incidentally that one perceives this, because being the son of Diares is an accident of the white thing which one perceives. Hence, too, one is in no way affected by the perceptible object qua such. But of the objects perceptible in themselves, the proper objects are perceptible in the strict sense, and it is to these that the essence of each sense is by nature adapted.

Book II, Chapter 7

That, then, of which there is sight is the visible. The visible is colour, and also something which can be described in words but happens to have no name; what we mean will become especially clear as we proceed. For the visible is colour, and colour is that which lies upon what is visible in itself — "in itself" not in respect of its definition, but because it has in itself the cause of its being visible. Every colour is capable of setting in movement that which is transparent in actuality, and this is its very nature. That is why it is not visible without light, but the proper colour of each thing is visible in light. Hence we must first say what light is.

There is, then, something transparent. By "transparent" I mean that which is indeed visible, yet not visible in itself, to speak without qualification, but by reason of a colour belonging to something else. Of this kind are air and water and many solid bodies; for it is not qua water nor qua air that they are transparent, but because there is a certain nature present in both of them which is one and the same, and which is present also in the eternal body above. Light is the actuality of this, of the transparent qua transparent; and where this is present only potentially, there darkness also is. Light is, as it were, the colour of the transparent, whenever the transparent is made actually transparent by fire or by something of the kind that the body above is; for to this too one and the same thing belongs.

What the transparent is, then, and what light is, has been stated: that light is neither fire, nor body in general, nor an effluence from any body (for in that way too it would be a kind of body), but the presence of fire, or of something of that kind, in the transparent; for it is not possible for two bodies to be in the same place at the same time. Light seems to be the contrary of darkness; but darkness is the privation of such a state from the transparent, so that it is clear that the presence of this state is light. And Empedocles was not right, nor was anyone else who has spoken in that way, in saying that light travels and comes to be at some moment between the earth and that which surrounds it, without our noticing it. For this is contrary both to the truth reached by argument and to the appearances; for over a small distance it might indeed escape our notice, but that it should escape notice all the way from east to west is far too great a thing to postulate.

What is receptive of colour is the colourless, and what is receptive of sound is the soundless. The colourless is the transparent, and the invisible, or what is only barely seen, such as the dark seems to be. And such is the transparent — not when it is transparent in actuality, but when it is so potentially; for it is one and the same nature that is at one time darkness and at another light.

Not all things are visible in light, but only the proper colour of each thing; for some things are not seen in the light, yet in the dark they produce perception, such as the things that appear fiery and shining (these have no one name that covers them), for instance fungus, horn, and the heads of fishes, their scales and their eyes. But in none of these is its own proper colour seen. For what reason these are seen is another matter; at present this much is clear, that what is seen in light is colour. That is why it is not seen without light; for this was precisely what it was for colour to be colour, namely to be capable of setting in movement that which is transparent in actuality; and the actuality of the transparent is light.

There is clear evidence of this: for if one places what has colour upon the eye itself, one will not see it. Rather, the colour sets in movement the transparent, for instance the air, and by this, since it is continuous, the sense-organ is set in movement. For Democritus does not speak rightly on this point, when he supposes that if the intervening space were to become void, one would see distinctly even if there were an ant in the sky; for this is impossible. For seeing comes about through the perceptive faculty being in some way acted upon; now it is impossible that it should be acted upon by the seen colour itself, so it remains that it is acted upon by the medium, and hence there must necessarily be something in between. But if the intervening space became void, then, so far from things being seen distinctly, nothing at all would be seen.

The reason, then, why colour must be seen in light has been stated. Fire is seen in both, both in darkness and in light, and this necessarily; for it is by fire that the transparent becomes transparent. And the same account holds of sound and of smell as well; for none of them produces perception by touching the sense-organ, but rather the medium is set in movement by the smell and by the sound, and by the medium each of the two sense-organs is moved. But when one places the sounding or the smelling object upon the sense-organ itself, it will produce no perception at all. With touch and taste the case is similar, though it does not appear to be so; and for what reason will become clear later.

The medium of sounds is air, but the medium of smell has no name. For there is indeed a certain affection common to air and to water, and as the transparent is to colour, so is that which is present in both of these to that which has smell; for water animals too are evidently possessed of the perception of smell. But man, and those land animals which breathe, are unable to smell when they are not breathing. The reason for these things too will be stated later.