De Anima

The object, then, of sight is the visible: what is visible is colour and something besides which can be described, though it has no name. What we mean will best be made clear as we proceed. The visible, then, is colour. Now colour is that with which what is visible in itself is overlaid: and, when I say in itself, I do not mean what is visible by its essence or form, but what is visible because it contains within itself the cause of visibility, namely, colour. But colour is universally capable of exciting change in the actually transparent, that is, in light; this being, in fact, the true nature of colour. Hence colour is not visible without light, but the colour of each object is always seen in light. And so we shall have first to explain what light is.

There is, then, we assume, something transparent; and by this I mean that which, though visible, is not properly sneaking, visible in itself, but by reason of extrinsic colour. Air, water and many solid bodies answer to this description. For they are not transparent qua air or qua water,

but because there is a certain natural attribute present in both of them which is present also in the eternal body on high. Light is the actuality of this transparent. But where the transparent is only potentially present, there darkness is actually. Light is a sort of colour in the transparent when made transparent in actuality by the agency of fire or something resembling the celestial body: for this body also has an attribute which is one and the same with that of it. What the transparent is, and what light is, has now been stated; namely, that corporeal. + it is neither fire nor body generally nor an effluence from any body (for even then it would still be a sort of body), but the presence of fire or something fiery in the transparent. For it is impossible for two bodies to occupy the same space at the same time.

Light is held to be contrary to darkness. But darkness is absence from the transparent of the quality above described: so that plainly light is the presence of it. Thus Empedocles and others who propounded the same view are wrong when they represent light as moving in space and arriving at a given point of time between the earth and that which surrounds it without our perceiving its motion. For this contradicts not only the clear evidence of reason, but also the facts of observation: since, though a movement of light might elude observation within a short distance, that it should do so all the way from east to west is too much to assume.

It is that which is colourless which is receptive of colour, as it is that which is soundless which is receptive of sound. And the transparent is colourless, and so is the invisible or the dimly visible which is our idea of the dark. Such is the transparent medium, not indeed when it is in actuality, but when potentially transparent. For it is the same natural attribute which is at one time darkness and at another time light. It is not everything visible which is visible in light, but only the proper colour of each thing. Some things, indeed, are not seen in daylight, though they produce sensation in the dark: as, for example, the things of fiery and glittering appearance, for which there is no one distinguishing name, like fungus, horn,

the heads, scales and eyes of fishes. But in no one of these cases is the proper colour seen. Why these objects are seen must be discussed elsewhere. At present this much is clear, that the object seen in light is colour, and this is why it is not seen without light. For the very quiddity of colour is, as we saw, just this, that it is capable of exciting change in the operantly transparent medium: and the activity of the transparent is light. There is clear evidence of this. If you lay the coloured object upon your eye, you will not see it. On the contrary, what the colour excites is the transparent medium, say, the air, and by this, which is continuous, the sense-organ is stimulated. For it was a mistake in Democritus to suppose that if the intervening space became a void, even an ant would be distinctly seen, supposing there were one in the sky. That is impossible. For sight takes place through an affection of the sensitive faculty. Now it cannot be affected by that which is seen, the colour itself: therefore it can only be by the intervening medium: hence the existence of some medium is necessary. But, if the intermediate space became a void, so far from being seen distinctly, an object would not be visible at all.

We have explained the reason why colour must be seen in light. Fire is visible both in light and in darkness: and necessarily so, for it is owing to fire that the transparent becomes transparent. The same argument holds for sound and odour. For no sound or scent produces sensation by contact with the sense-organ: it is the intervening medium which is excited by sound and odour and the respective sense-organs by the medium. But, when the body which emits the sound or odour is placed on the sense-organ itself, it will not produce any sensation. The same holds of touch and taste, although it appears to be otherwise. The reason for this will be seen hereafter. The medium for sounds is air, that for odour has no name. For there is assuredly a common quality in air and water, and this quality, which is present in both, stands to the body which emits odour in the same relation as the transparent

to colour. For the animals that live in water also appear to have the sense of smell. But man and the other land-animals which breathe are unable to smell without inhaling breath. The reason for this, too, must be reserved for future explanation.