418a26-b3
1. What is visible (ὁρατόν) is, say, color.
2. Color is that which is on (ἐπὶ) what is in itself visible, say X.
3. X is in itself visible not by definition but because X has in itself the explanation of being visible.
4. Every color is capable of setting in motion that which is actually transparent.
5. 4. is the nature of color.
6. Therefore, color is not visible without light. (How come?)
There is one missing step between premises 3 and 4. So we need to supply one more premise:
3’ Color is capable of setting in motion because it has the explanation of being visible. (from 2, 3)
How does Aristotle get the conclusion 6? Perhaps we should supply two more premises:
5’ But color is not visible without x or without the explanation of being visible. (from 2, 3, 4)
5’’ The explanation of being visible is something transparent or light. (But it seems to me that the explanation of being visible should be understood as light instead of transparency, since transparency is not visible in itself, see the following argument)
418b3-13
1. Light is something transparent.
2. If a is transparent, a is visible. Nevertheless, a is not visible in itself, but visible because of the color of b.
3. The instances of a are solid bodies such as air and water.
4. Water is not transparent insofar as it is water, but because it has a certain nature that is the same in them both (ἐν τούτοις ἀμφοτέροις) and also in the eternal heavenly bodies. (from 2?)
5. Light is the energeia of this [sc. transparency or the transparent thing], the transparent insofar as it is transparent.
6. Potentially, in things in which there is transparency, there is also darkness.
7. Light is in actuality because of fire or this sort of things such as heavenly bodies.
8. Therefore, light is a sort of color of the transparent. (from 2,5,7)
What des “them both” (ἐν τούτοις ἀμφοτέροις) refer to in premise 4?
What is “the certain nature” in premise 4? It seems to refer to “a is visible because of the color of b” (i.e., premise 2)
418b13-20
9. The transparent and light are not fire nor body generally, nor an effluence from anybody (for it would be a body in that case as well), but the presence (παρουσία) of fire or this sort of thing in the transparent. (from 7?)
10. The reason why the transparent and light cannot be bodies is that it is impossible for two bodies to be in the same place at the same time. (10 explains 9)
11. Light is opposite to darkness in the sense that darkness is the privation of this such state [sc. the actualized state?] from the transparent.
12. Therefore, light is the presence of the transparent. (from 9,11)
