De Anima

Inasmuch as we perceive that we see and hear, it must either be by sight or by some other sense that the percipient perceives that he sees. But, it may be urged, the same sense which perceives sight will also perceive the colour which is the object of sight. So that either there will be two senses to perceive the same thing or the one sense, sight, will perceive itself. Further, if the sense perceiving sight were really a distinct sense, either the series would go on to infinity or some one of the series of senses would perceive itself. Therefore it will be better to admit this of the first in the series. Here, however, there is a difficulty. Assuming that to perceive by sight is to see and that it is colour or that which possesses colour which is seen, it may be argued that, if you are to see that which sees, that which in the first instance sees, the primary visual organ, will actually have colour. Clearly, then, to perceive by sight does not always mean one and the same thing. For, even when we do not see, it is nevertheless by sight that we discern both darkness and light, though not in the same manner. Further, that which sees is in a manner coloured. For the sense-organ is in every case receptive of the sensible object without its matter. And this is why the sensations and images remain in the sense-organs even when the sensible objects are withdrawn.

Now the actuality of the sensible object is one and the same with that of the sense, though, taken in the abstract, sensible object and sense are not the same. I mean, for example, actual sound and actual hearing are the same: for it is possible to have hearing and yet not hear; again, that which is resonant is not always sounding. But when that which is capable of hearing operantly hears and that which is capable of sounding sounds, the actual hearing and the actual sound occur simultaneously, and we might, if we pleased, call them audition and resonance respectively. If, then, motion, action and passivity reside in that which is acted upon, then of necessity it is in the potentiality of hearing that there is actual sound and there is actual hearing. For the activity of agent and movent comes into play in the patient; and this is why that which causes motion need not itself be moved. The actuality of the resonant, then, is sound or resonance, and the actuality of that which can hear is hearing or audition, hearing and sound both having two meanings. The same account may be given of the other senses and their objects. For, just as acting and being acted upon are in the subject acted upon and not in the agent, so also the actuality of the sensible object and that of the sensitive faculty will be in the percipient subject. But in some cases both activities have a name; for example, resonance and audition: in other cases one or the other has no name. Thus, while the actuality of sight is called seeing, that of colour has no name; and, while the actuality of the taste-faculty is called tasting, that of the flavour has no name. Now, as the actuality of the object and that of the faculty of sense are one and the same, although taken in the abstract they are different, hearing and sound thus understood as operant must simultaneously cease to be or simultaneously continue in being, and so also with flavour and taste, and similarly with the other senses and their objects: but when they are understood as potentialities, there is no such necessity. On this point the earlier natural philosophers were in error, when they supposed that without seeing there was neither white nor black, and without tasting no flavour. Their statement is in one sense true, in another false. For the terms sensation and sensible thing are ambiguous. When they mean the actual sensation and the actual sensible thing, the statement holds good: when they mean potential sensation and potential sensible, this is not the case. But our predecessors used terms without distinguishing their various meanings.

If, then, concord consists in a species of vocal sound, and if vocal sound and hearing are in one aspect one and the same, [though in another aspect not the same], and if concord is a proportion, it follows that hearing must also be a species of proportion. And this is the reason why hearing is destroyed by either excess, whether of high pitch or of low. And similarly, in the case of flavours, excess destroys the taste, and in colours excessive brightness or darkness destroys the sight, and so with smell, whether the excessive odour be agreeable or pungent. All this implies that the sense is a proportion. Hence sensibles are, it is true, pleasurable when they are brought into the range of this proportion pure and unmixed; for example, the shrill, the sweet, the salt: in that case, I say, they are pleasurable. But, speaking generally, that in which ingredients are blended is pleasurable in a higher degree, accord more pleasurable to the ear than high pitch or low pitch alone¹, and to touch that which admits of being still further heated or cooled. The due proportion constitutes the sense, while objects in excess give pain or cause destruction.

Now each sense is concerned with its own sensible object, being resident in the organ, qua sense-organ, and judges the specific differences of its own sensible object. Thus sight pronounces upon white and black, taste upon sweet and bitter, and so with the rest. But, since we compare white and sweet and each of the sensibles with each, what in fact is it by means of which we perceive the difference between them? It must be by sense, for they are sensibles. And thus it is clear that the flesh is not the ultimate organ of sense; for, if it were, it would be necessary that that which judges should judge by contact with the sensible object. Nor indeed can we with separate organs judge that sweet is different from white, but both objects must be clearly presented to some single faculty. For, if we could, then the mere fact of my perceiving one thing and your perceiving another would make it clear that the two things were different. But the single faculty is required to pronounce them different, for sweet and white are pronounced to be different. It is one and the same faculty, then, which so pronounces. Hence, as it pronounces, so it also thinks and perceives. Clearly, then, it is not possible with separate organs to pronounce judgment upon things which are separate: nor yet at separate times, as the following considerations show. For, as it is one single faculty which pronounces that good and bad are different, so when it judges "A is different from B" it also judges "B is different from A" (and in this case the "when" is not accidental; I mean, accidental in the sense in which I may now say "Such and such things are different" without saying that they are different now. On the contrary, it pronounces now and pronounces that A and B are different now). That which judges judges, then, instantaneously and hence as an inseparable unit in an inseparable time. But, again, it is impossible for the same thing, in so far as indivisible and affected in indivisible time, to be moved at the same instant with contrary motions. For, if the object be sweet, it moves sense or thought in such and such a way, but what is bitter moves it in a contrary way, and what is white in a different way.

Is, then, that which judges instantaneous in its judgment and numerically undivided and inseparable, although separated logically? Then it is in a certain sense that which is divided which perceives divided objects; in another sense it is qua indivisible that the divided perceives them: that is to say, logically it is divisible, locally and numerically it is indivisible. Or is this impossible? For the same indivisible unity, though in potentiality each of two opposites, in the order of thought and being is not so, but in actual operation is divided: it is impossible that it should be at the same time both white and black, and hence impossible that it should receive at the same time the forms of white and black, if reception of the forms constitutes sensation and thought. Rather is the case parallel to that of the point, as some describe it, which is divisible in so far as it is regarded as one or two. Well then, in so far as the faculty which judges is indivisible, it is one and judges instantaneously; but, in so far as it is divisible, it is not one, for it uses the same point at the same time twice. So far as it treats the boundary-point as two, it passes judgment on two separate things with a faculty which in a manner is separated into two; so far as it treats the point as one, it passes judgment on one thing, and that instantaneously. So much, then, for the principle in virtue of which we call the animal capable of sensation.