The visible is that for which vision is the sense, and the visible is both colour and something which is describable by words, although it happens to be without a name; but our meaning will become clear to those who accompany us in the inquiry. The visible is colour, and colour is that which is upon something visible in itself; and this something is visible, not only after its appellation but, because it has in itself the cause of being visible. All colour is motive of the diaphanous, in activity, and to be so motive is the nature of colour. On which account nothing is visible without light, but the colour of each object is visible in the light; and we must, therefore, first say what light is. There is a something diaphanous, and I call diaphanous what is visible, and yet not visible, strictly speaking, in itself, but made visible by colour, which is foreign to it. Such is air, and water, and many solid bodies; yet neither air nor water, as air or water, is diaphanous, but the same nature is present in both those elements, which is in the eternal supernal body. Light is the active state of that same diaphanous, in so far as it is diaphanous, and darkness is the same in its state of potentiality. But light is the colour, as it were, of the diaphanous, when made diaphanous in reality by fire, or other such element as the supernal body; for to it belongs a something which is identical with fire. We have thus said what is the diaphanous and what light, and have shewn that neither of them is fire, nor a body, strictly speaking, nor an emanation from a body, (as, in that case, they would be corporeal), but that they are the presence in the diaphanous of fire or something analogous to fire, since two bodies cannot possibly coexist in one and the same body.
Light seems to be the opposite to darkness; and as darkness is the absence of a particular state of the diaphanous, it is evident that the presence of that state must be light.
Thus Empedocles, or whoever else may have held the same opinion, was wrong in supposing that light was transported and manifested, without our consciousness, between the Earth and surrounding space; for the opinion is opposed as well to sound conclusion as to observation of the phenomenon. If the interval were small, the fact might, indeed, escape us; but, extended as it is from the East to the West, the postulate is too extravagant to be admitted.
Now that which is without colour is receptive of colour, as that which is without sound is receptive of sound; and that which is without colour is the diaphanous and the invisible or scarcely visible, such as darkness seems to be. Such too is the diaphanous; but then it is the diaphanous, not in potentiality but, in reality; for the same nature is sometimes darkness and sometimes light. But all objects are not visible in light, as there are some of which the peculiar colour only of each is visible; for some, not visible in light, produce sensation in the dark, as certain fiery brilliant appearances (which have no special appellation,) which emanate from fungi, horn, scales and eyes of fishes, but the peculiar colour is not seen of any one of those objects. It is foreign to our present purpose to explain how such objects become visible; but this much is manifest, that it is colour which is visible in light. Therefore, without light colour is not visible; for it is an essential property of colour to be motive of the diaphanous in activity, and the reality of the diaphanous is light. As proof of this, if any coloured object be placed over the sight, the object will not be seen, and yet there is colour, which is motive of the diaphanous, the air, that is, and, by its being continuous between the object and the sense, it is able to give motion to the visual organ. Thus, Democritus was wrong in thinking that if the medium were a void, vision would be so accurate as to render an ant visible in the sky. The opinion, in fact, involves an impossibility; for vision is produced by some kind of impression upon the visual organ, and as this cannot possibly be effected by the colour which is visible, there remains only that it must be by the medium, and thus a medium there must be; so that if there were a void, vision would be, not to say inaccurate but, altogether precluded.
It has thus then been said why colour must be visible in the light; but fire is visible both in darkness and in light, and necessarily so, since it is by fire that the diaphanous becomes diaphanous. The same reasoning holds good for sound and for odour, as nothing sonorous or odorous can produce sensation when in immediate contact with the sentient organ; but by odour or sound the medium is set in motion, and by it the organ is moved. Thus, when any thing sonorous or odorous is placed immediately upon the sentient organ, no sensation is given out; and this is the case with the sense of Touch, although less evidently so; but the cause of this shall be explained hereafter. The air is the medium for sounds, while that for odour has no special appellation, for there is a particular impression common to air and water; and what the diaphanous is to colour that which is in those elements is to odorous bodies, as aquatic animals appear to be sensible of odours. But neither man nor animals which breathe can smell without inspiring; and the cause of this shall be spoken of hereafter.