Light in the Causal Order According to St. Thomas

Abstract: Although a great deal of attention has been given to Aquinas's understanding of the light of the intellect, little is made of his understanding of physical light, and even less of how he conceives of light's role in the order of the physical cosmos. One suspects that the reason for this scholarly reticence is an underlying suspicion that Aquinas has nothing of relevance to say when science has come so far since the thirteenth century. This essay aims both to make a start toward filling the lacuna and toward showing the ways in which Aquinas's views about physical light—both its nature and its causal role in nature—are worth taking seriously. I confirm the latter point indirectly by explaining out how contemporary science's view of light as an electromagnetic wave bears a certain resemblance to Aristotle and Aquinas's definition of light as the "act of the transparent" and the way this act functions as a quasi-universal instrumental cause.