6. We return, then, to the question whether there could be light if there were no air, the sun illuminating corporeal surfaces across an intermediate void which, as things are, takes the light accidentally by the mere fact of being in the path. Supposing air to be the cause of the rest of things being thus affected, the substantial existence of light is due to the air; light becomes a modification of the air, and of course if the thing to be modified did not exist neither could be modification.
The fact is that primarily light is no appanage of air, and does not depend upon the existence of air: it belongs to every fiery and shining body, it constitutes even the gleaming surface of certain stones.
Now if, thus, it enters into other substances from something gleaming, could it exist in the absence of its container?
There is a distinction to be made: if it is a quality, some quality of some substance, then light, equally with other qualities, will need a body in which to lodge: if, on the contrary, it is an activity rising from something else, we can surely conceive it existing, though there be no neighbouring body but, if that is possible, a blank void which it will overleap and so appear on the further side: it is powerful, and may very well pass over unhelped. If it were of a nature to fall, nothing would keep it up, certainly not the air or anything that takes its light; there is no reason why they should draw the light from its source and speed it onwards.
Light is not an accidental to something else, requiring therefore to be lodged in a base; nor is it a modification, demanding a base in which the modification occurs: if this were so, it would vanish when the object or substance disappeared; but it does not; it strikes onward; so, too [requiring neither air nor object] it would always have its movement.
But movement, where?
Is space, pure and simple, all that is necessary?
With unchecked motion of the light outward, the material sun will be losing its energy, for the light is its expression.
Perhaps; and [from this untenable consequence] we may gather that the light never was an appanage of anything, but is the expressive Act proceeding from a base [the sun] but not seeking to enter into a base, though having some operation upon any base that may be present.
Life is also an Act, the Act of the soul, and it remains so when anything- the human body, for instance- comes in its path to be affected by it; and it is equally an Act though there be nothing for it to modify: surely this may be true of light, one of the Acts of whatever luminary source there be [i.e., light, affecting things, may be quite independent of them and require no medium, air or other]. Certainly light is not brought into being by the dark thing, air, which on the contrary tends to gloom it over with some touch of earth so that it is no longer the brilliant reality: as reasonable to talk of some substance being sweet because it is mixed with something bitter.
If we are told that light is a mode of the air, we answer that this would necessarily imply that the air itself is changed to produce the new mode; in other words, its characteristic darkness must change into non-darkness; but we know that the air maintains its character, in no wise affected: the modification of a thing is an experience within that thing itself: light therefore is not a modification of the air, but a self-existent in whose path the air happens to be present.
On this point we need dwell no longer; but there remains still a question.