Book III, Chapter 3

Since the soul is defined chiefly by two differentiae — by movement in respect of place, and by thinking, judging, and perceiving — and since thinking and understanding are held to be a kind of perceiving (for in both of these the soul judges something and comes to know something of the things that are); and since the ancients, indeed, say that understanding and perceiving are the same, as Empedocles has said, […], and elsewhere, […] — and Homer's verse, […], means the same as these. For all of them suppose that thinking is bodily, just as perceiving is, and that we both perceive and understand like by like, as we determined in our discussions at the outset. Yet they ought at the same time to have spoken about error as well; for error is more proper to living creatures, and the soul continues longer in this state. Hence it must be either, as some say, that all appearances are true, or that contact with the unlike is error; for this is the contrary of knowing like by like. But it is held that error, and knowledge of contraries, are one and the same.

That perceiving and understanding are not the same, then, is evident; for all animals share in the one, but only a few in the other.

Nor yet is thinking the same as perceiving — thinking, in which there is the right and the not-right, the right being practical wisdom and knowledge and true opinion, the not-right their contraries. Neither is this the same as perceiving; for perception of the special objects is always true, and belongs to all animals, whereas it is possible also to think discursively in a false way, and this belongs to nothing which does not also have reason. For imagination (phantasia) is other than both perception and thought; and imagination itself does not come about without perception, nor is there judgement without imagination.

That this thinking and judgement are not the same is evident. For this affection lies in our own power, whenever we wish (for it is possible to bring something before our eyes, as those do who set things out and form images in the mnemonic systems), whereas holding an opinion is not in our power; for it must be either false or true. Again, whenever we form the opinion that something is dreadful or fearful, we are at once affected accordingly, and likewise if it is something encouraging; but in the case of imagination we are just as we should be if we were looking at dreadful or encouraging things in a picture. There are also differences within judgement itself: knowledge, opinion, practical wisdom, and their contraries; but let the difference between these be the subject of another discussion.

As for thinking, since it is other than perceiving, and of it one part is held to be imagination and another judgement, we must speak about the other after we have determined about imagination.

If, then, imagination is that in virtue of which we say that an image comes to be in us — and not anything we speak of metaphorically — is it some one faculty or state among those in virtue of which we judge and are either right or in error? Such are perception, opinion, knowledge, intellect.

That it is not perception is clear from the following. Perception is either a faculty or an activity, as sight and seeing are; but something appears even when neither of these is present, as in dreams. Again, perception is always present, but imagination is not. And if they were the same in actuality, imagination could belong to all the beasts; but it seems that it does not — not, for instance, to the ant, the bee, or the grub. Again, perceptions are always true, whereas imaginings turn out for the most part false. Further, when we are exercising our sense accurately upon the object of perception, we do not say "this appears to us to be a man"; rather it is when we do not perceive distinctly that the perception is either true or false. And, as we said before, visions appear even to those whose eyes are shut.

But neither will imagination be any of those things which are always true, such as knowledge or intellect; for imagination may also be false.

It remains, then, to see whether it is opinion; for opinion comes to be both true and false. But conviction follows upon opinion (for one who holds an opinion cannot fail to be convinced of what seems to him to be so), while no beast has conviction, though many have imagination. Again, upon every opinion there follows conviction, upon conviction being persuaded, and upon persuasion reason; but while some of the beasts have imagination, none has reason.

It is evident, therefore, that imagination could not be opinion together with perception, nor opinion through perception, nor a combination of opinion and perception, both for these reasons and because it is plain that the opinion would be of nothing other than that of which there is also perception. I mean that the combination of the opinion of white and the perception of white would be imagination; for surely it would not be a combination of the opinion of the good and the perception of white. To imagine, then, would be to hold an opinion about just that which one perceives, and not incidentally. But things also appear falsely about which one has at the same time a true judgement: for instance, the sun appears a foot across, yet one is convinced that it is larger than the inhabited world. It follows, then, either that one has thrown away the true opinion one had, although the thing itself is preserved and one has neither forgotten it nor been persuaded out of it; or, if one still has it, the same opinion must be both true and false. But it would have become false only if the thing had changed without one's noticing. Imagination, therefore, is neither any one of these things nor composed of them.

But since, when one thing has been moved, another can be moved by it; and since imagination is held to be a kind of movement, and not to come about apart from perception, but in things that perceive and about things of which there is perception; and since it is possible for a movement to be produced by the activity of perception, and this movement must be like the perception — this movement would be one that cannot occur without perception, and cannot belong to things that do not perceive; and in virtue of it the thing that has it both does and undergoes many things; and it is both true and false.

This last comes about for the following reasons. Perception of the special objects is true, or has the least possible falsehood. Second comes the perception that these things are attributes of something; and here already it is possible to be deceived — for that it is white one is not deceived, but whether the white thing is this or something else, one is deceived. Third is the perception of the common objects and of those attendant on the incidental objects to which the special objects belong; I mean, for instance, movement and magnitude, which are attributes of perceptible things, and it is about these above all that one may already be deceived in perceiving. And the movement produced by the activity of perception will differ according to which of these three kinds of perception it comes from. The first is true so long as the perception is present, but the others may be false whether it is present or absent, and especially when the perceptible object is far off.

If, then, the things described have no other character than imagination, and imagination is what has been said, imagination will be a movement produced by perception in actual operation. And since sight is above all perception, imagination has taken its name from light (phaos), because without light it is not possible to see. And because imaginations persist and are like perceptions, animals do many things in accordance with them — some because they have no intellect, as the beasts do, and others because their intellect is at times overclouded by passion or diseases or sleep, as with men.

Let this, then, be all that is said about imagination, what it is and why it is.

Book III, Chapter 4

Concerning that part of the soul by which the soul both knows and understands — whether it is separable, or not separable in magnitude but only in definition — we must consider what differentia it has, and how thinking ever comes about. If, then, thinking is like perceiving, it would be either a being acted upon in some way by the object of thought, or something else of this kind. It must, then, be unaffected, yet receptive of the form, and potentially such as the form is, though not identical with it; and intellect (nous) must stand to the objects of thought as the perceptive part stands to the objects of perception.

Necessarily, then, since it thinks all things, it must be unmixed, as Anaxagoras says, in order that it may master them — that is, in order that it may know them; for anything foreign that shows itself alongside hinders and screens it off. Hence it too has no nature of its own other than this, that it is potential. So what is called the intellect of the soul (and by intellect I mean that by which the soul thinks discursively and judges) is in actuality none of the things that are before it thinks. Hence it is not reasonable that it should be mixed with the body; for then it would come to have some quality, cold or hot, or there would even be some organ for it, as there is for the perceptive part; but in fact there is none. And so those speak well who say that the soul is the place of forms — except that it is not the whole soul but the thinking soul, and that it is the forms not in actuality but potentially.

That the impassivity of the perceptive part and of the thinking part are not alike is evident in the case of the sense-organs and of perception. For the sense cannot perceive after an excessively strong object of perception — it cannot perceive sound, for instance, after loud sounds, nor see nor smell after strong colours and odours. But when the intellect has thought something that is very much an object of thought, it thinks the inferior objects not less but even more; for the perceptive part is not without a body, whereas the intellect is separable.

But when it has come to be each thing in the way in which one who actually knows is said to know (and this comes about when he is able to exercise the power through himself), even then it is in a way still in potentiality, though not in the same way as before he learned or discovered; and then it is able to think itself.

Since magnitude and the essence of magnitude are different, and water and the essence of water — and so too in many other cases, though not in all, for in some cases the two are the same — it judges flesh and the essence of flesh either by something different, or by the same thing differently disposed. For flesh does not exist without matter, but is, like the snub, a this in a this. With the perceptive part, then, it judges the hot and the cold, and the things of which flesh is a certain ratio; but it judges the essence of flesh by something else, either something separate, or standing to the former as the bent line stands to itself when it has been straightened out.

Again, in the case of things that exist in abstraction, the straight is like the snub, for it involves something continuous; but its essence, if the essence of straight and the straight are different, is something else — let it be duality. It judges it, therefore, by something different, or by the same thing differently disposed. And in general, therefore, as things are separable from matter, so it is also with what concerns the intellect.

But one might raise a difficulty: if the intellect is simple and unaffected and has nothing in common with anything, as Anaxagoras says, how will it think, if thinking is a being acted upon in some way? For it is in so far as something is common to two things that the one is held to act and the other to be acted upon. Again, there is the question whether the intellect is itself an object of thought. For either intellect will belong to the other things, if it is not itself an object of thought in virtue of something else, and if what is thought is one in form; or it will have something mixed in it which makes it an object of thought like the rest.

Or rather, being acted upon in virtue of something common has been distinguished earlier: the intellect is in a way potentially the objects of thought, but is nothing in actuality before it thinks. It must be as in a writing-tablet on which nothing actually stands written; which is just what happens in the case of the intellect. And it is itself an object of thought, just as the objects of thought are. For in the case of things without matter, that which thinks and that which is thought are the same; for contemplative knowledge and what is knowable in that way are the same. But the cause of its not always thinking must be considered. In things which have matter, on the other hand, each of the objects of thought is present potentially. Hence intellect will not belong to them — for the intellect is a potentiality for such things without matter — whereas the object of thought will belong to it.