Book III, Chapter 1

That there is no sense besides the five — I mean sight, hearing, smell, taste and touch — one might be convinced from the following considerations. For if we in fact have perception of everything of which touch is the sense (since all the affections of the tangible qua tangible are perceptible to us by touch), then necessarily, if some sense is wanting to us, some sense-organ must also be wanting to us. And all the things we perceive by touching them directly are perceptible by touch, which we happen to possess; while all that we perceive through media, and not by touching them directly, we perceive by means of the simple bodies, I mean air and water.

And the case stands thus. If through a single medium several sensible objects are perceived which differ from one another in kind, then whoever has a sense-organ made of such a thing must be percipient of both; for instance, if the sense-organ is made of air, and air is the medium both of sound and of colour. But if there are several media of the same sensible object — as both air and water are media of colour, for both are transparent — then even one who has only one of them will perceive whatever is perceived through both.

Now of the simple bodies, the sense-organs are composed of these two only, air and water; for the pupil is of water, hearing is of air, and smell of one or other of these. Fire is either the organ of none, or else common to all, since nothing is capable of perception without heat. Earth is the organ of none, or else is mixed in touch above all in a special way. Hence it would follow that there is no sense-organ apart from water and air; and these organs some animals actually possess even now. All the senses, therefore, are possessed by animals that are neither imperfect nor maimed; for even the mole is seen to have eyes beneath its skin. So that, unless there is some other body, and some affection belonging to none of the bodies here, no sense can be wanting.

But further, neither can there be any special sense-organ for the common sensibles, which we perceive by each sense incidentally — such as movement, rest, shape, magnitude, number and unity. For all these we perceive by movement: magnitude, for instance, by movement, and hence shape as well, since shape is a kind of magnitude; and what is at rest by its not being moved; and number by the negation of continuity and by the special sensibles, since each sense perceives one thing. Hence it is clear that it is impossible for there to be a special sense of any of these, of movement, for example; for in that case it would be just as we now perceive the sweet by sight. And this comes about because we happen to have a sense of both, in virtue of which we recognize them when they coincide. Otherwise we should in no way perceive such a thing except incidentally — just as we perceive the son of Cleon not because he is the son of Cleon, but because he is white, and it is incidental to this white thing to be the son of Cleon.

But of the common sensibles we already have a common sense, and not incidentally; there is not, therefore, any special sense of them, for otherwise we should in no way perceive them except in the way described, in which we are said to see the son of Cleon.

The senses perceive one another's special objects incidentally, not qua being the several senses that they are, but qua being one, whenever the perception occurs at the same time about the same object — as, for instance, that bile is bitter and yellow. For it is certainly not the business of some further sense to say that the two are one; and hence it is also deceived, and if a thing is yellow it supposes it to be bile.

One might inquire for what purpose we have several senses and not one only. Is it so that the accompanying common sensibles may less escape our notice — movement, for instance, and magnitude and number? For if there were sight alone, and that of white, they would have escaped us more, and everything would have seemed to be the same, because colour and magnitude accompany one another. But as it is, since the common sensibles are found in another sensible object as well, this makes it clear that each of them is something distinct.

Book III, Chapter 2

Since we perceive that we see and hear, it must be either by sight that one perceives that one sees, or by some other sense. But in that case the same sense will be of sight and of the colour that underlies it; so that either there will be two senses of the same object, or the sense will be of itself. Again, if the sense of sight were a different sense, either there will be an infinite regress, or some sense will be of itself; so that we had better admit this in the first case.

But there is a difficulty. For if to perceive by sight is to see, and what is seen is colour or that which has colour, then, if one is to see that which sees, that which sees primarily will itself have colour. It is evident, then, that to perceive by sight is not a single thing; for even when we do not see, it is by sight that we judge both darkness and light, though not in the same way. Again, that which sees is in a way coloured, since each sense-organ is receptive of the sensible object without the matter. Hence, even when the sensible objects have departed, sensations and appearances (phantasiai) are present in the sense-organs.

The actuality of the sensible object and that of the sense is one and the same, but their being is not the same. I mean, for instance, sound in actuality and hearing in actuality; for one may have hearing and yet not hear, and that which has sound is not always sounding. But whenever that which can hear is active and that which can sound is sounding, then the hearing in actuality and the sound in actuality come about together — and one might call the one hearkening and the other sounding.

If, then, the movement, that is, the acting and the being acted upon, resides in that which is acted upon, both the sound and the hearing in actuality must reside in that which is such potentially; for the actuality of the agent and the mover comes to be in the patient. Hence it is not necessary that what moves should itself be moved. The actuality of that which can sound, then, is sound or sounding, and that of what can hear is hearing or hearkening; for hearing is twofold, and sound is twofold.

The same account holds also of the other senses and sensible objects. For just as the acting and the being acted upon are in that which is acted upon and not in the agent, so too the actuality of the sensible object and that of the sensitive faculty are in the sensitive faculty. But in some cases both have received a name, as sounding and hearkening; in others one of the two is nameless, for the actuality of sight is called seeing, but that of colour is nameless, and the actuality of the faculty of taste is called tasting, but that of flavour is nameless.

Since the actuality of the sensible object and that of the sensitive faculty is one, while their being is different, hearing and sound as thus spoken of must perish and be preserved together, and so likewise flavour and taste and the rest; but for those spoken of potentially this is not necessary. The earlier natural philosophers did not state this well, holding that nothing was either white or black without sight, and no flavour without taste. For in one way they spoke rightly, in another not rightly. For since sense and the sensible object are each spoken of in two ways, some in potentiality and some in actuality, what they said holds of the latter, but does not hold of the former. But they spoke without qualification of things that are not spoken of without qualification.

But if concord is a kind of voice, and voice and hearing are in one way one and in another way not one and the same, and if concord is a ratio, then hearing too must be a certain ratio. And it is for this reason that each excess destroys hearing, both the high note and the low; and similarly among flavours excess destroys taste, and among colours what is exceedingly bright or exceedingly dark destroys sight, and in smell a strong odour, whether sweet or bitter — since the sense is a kind of ratio. Hence things are pleasant when, being pure and unmixed, they are brought into the ratio, as the sharp or the sweet or the salt; for then they are pleasant. But in general the mixed is more a concord than the high or the low note. And for touch the object is what can heat or cool; and the sense is the ratio, but excesses give pain or destroy.

Each sense, then, is of its underlying sensible object, residing in the sense-organ qua sense-organ, and it judges the differences of that underlying sensible object: sight, for instance, judges white and black, and taste sweet and bitter; and the same holds in the other cases too.

But since we also judge white and sweet, and each of the sensible objects, against one another, it is by something too that we perceive that they differ. And this must be by sense, for they are sensible objects. Whence it is also clear that the flesh is not the ultimate sense-organ; for it would then be necessary that what judges should judge by touching the object itself.

Nor, again, is it possible to judge by separate faculties that sweet is different from white, but both must be manifest to some one thing. For otherwise, even if I were to perceive the one and you the other, it would still be evident that they differ from one another; whereas it must be one and the same thing that says that they differ, since sweet is different from white. It is therefore the same thing that says this; and so, as it says, so also it thinks and perceives.

That it is impossible, then, to judge separate objects by separate faculties, is clear; and that it is impossible to do so in separate time is clear from the following. For just as it is the same thing that says that the good and the bad are different, so also, when it says of the one that it is different, it says the same of the other, and the "when" is not incidental. I mean, for instance, that I now say that they are different, but not that they are now different; rather it says thus: both now, and that they are different now. It judges, therefore, at one and the same time; and so the judging faculty is inseparable and judges in inseparable time.

But surely it is impossible that the same thing should be moved with contrary movements at the same time, insofar as it is undivided and in undivided time. For if the sweet moves the sense or the thought in this way, and the bitter in the contrary way, and the white in yet another way — is the judging faculty, then, at once numerically undivided and inseparable, but separated in its being?

In one way, then, it is qua divisible that it perceives divided things, but in another way it does so qua indivisible; for in its being it is divisible, but in place and in number indivisible. Or is this not possible? For potentially the same and indivisible thing is the contraries, but in its being it is not; rather it becomes divisible by being actualized, and it cannot be at once white and black. So neither can it be affected by their forms at once, if sensation and thinking are of such a kind. Rather it is like what some call a point, which, qua one and qua two, is in that respect also divisible.

Insofar, then, as it is indivisible, that which judges is one and judges at one time; but insofar as it is divisible, it uses the same point twice at the same time. Insofar, then, as it uses the point as two, it judges two things by the limit, and they are separate as being of things separate; but insofar as it uses it as one, it judges by one thing and at one time.

Concerning the principle, then, in virtue of which we say the animal is capable of perception, let it be determined in this way.