The word "light" originally signified what makes manifest to the sense of sight, its use being afterwards extended to whatever makes manifest, according to any mode of knowledge. In the first sense it is applied metaphorically to spiritual things; but, taken according to the ordinary use of language, as extended to all manifestation, it is used in a strict sense.
Since light is in bodies, it cannot be itself a body, or there would be two bodies in the same place; nor would it shine forth as it does, instantaneously, from east to west; for corporeal motion is not instantaneous. Moreover, were it a body, when the air is darkened by the absence of the sun it would be dissolved and take another form; which does not appear. And if any one should object that it is not dissolved, but carried round with the sun, how are we to explain that by the interposition of some opaque body round a candle the whole house may be darkened and yet the light will not accumulate round the candle.
Light is therefore an active quality consequent upon the substantial form of the sun; and the proof of this is that the effects of light differ according to the body which emits it.
Nor has the light only a subjective existence. For mental effects do not cause changes in external nature; while the sun's rays warm bodies. Neither is light the substantial form of the sun, for what is substantial form in one thing cannot be accidental form in another. Hence it is an active quality, consequent upon the sun.
The production of light is suitably assigned to the First Day; for, according to Augustine, the light of the angelic nature is to be here understood. According to others, light is first mentioned because it is a quality of the highest bodies, and by it the inferior communicate with the superior; also because all things are made manifest by light, nor can there be any day without it.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni