CHAPTER VII

SIGHT. ITS OBJECT

             THAT of which there is sight is the visible; and the visible is colour, and also something which, though it has no name, we can state descriptively. It will be evident what we mean when we have gone further into the matter.§ 399

             For the visible is colour, and it is this of which visibility is predicated essentially; not, however, by definition, but because it has in itself the cause of being visible. For every colour is a motivating force upon the actually transparent: this is its very nature. Hence nothing is visible without light; but by light each and every colour can be seen. Wherefore, we must first decide what light is.§§ 400-3

             There is, accordingly, something transparent. By transparent I mean that which is, indeed, visible, yet not of itself, or absolutely, but by virtue of concomitant colour. Air and water and many solids are such. But transparency does not depend on either air or water as such, but on the same quality being found in both, and in the eternal sphere above as well.§ 404

             Light is the act of this transparency, as such: but in potency this [transparency] is also darkness. Now, light is a kind of colour of the transparent, in so far as this is actualised by fire or something similar to the celestial body; which contains indeed something of one and the same nature as fire.§ 405

             We have then indicated what the transparent is, and what light is; that light is not fire or any bodily thing, nor any emanation from a body--[if it were this last,] it would be a sort of body, and so be fire or the presence of something similar in the transparent. 406

             For it is impossible for two bodies to exist in the same place at the same time.§ 407

             Light seems to be the contrary of darkness; and the latter is the privation of this quality in the transparent. So it is plain that the presence of this is light.§ 408

             Empedocles (or anyone else who may have said the same) was wrong when he said that light was borne along and extended between the earth and its envelope, unperceived by us. This is in contradiction alike to sound reasoning and to appearance. Such a thing might happen unobserved over a small space: but that it should remain unnoticed from the east to the west is a very extravagant postulate.§§ 409-26