The nature of light according to Thomas Aquinas

  • Book excerpt
  • 2800 words
  • Level: university

This discussion of light from the Summa Theologiae of Thomas Aquinas contrasts and compares in interesting ways with the modern understanding of light. For example, St. Thomas argues that light is not a body, which agrees with modern ideas. He also argues that light travels instantaneously, which does not agree with modern ideas. This excerpt has been selected by the Interdisciplinary Encyclopedia of Religion and Science (, which is edited by the Advanced School for Interdisciplinary Research, operating at the Pontifical University of the Holy Cross, Rome, and directed by Giuseppe Tanzella-Nitti. From

When speaking about light, faithful to his method, Thomas Aquinas’ starting point is terminology: he wants to clarify the use of the word “light” in all its different meanings, hoping to avoid misunderstanding, and this offering us a lesson in a scientific method which seems particularly valid even today (see a.1). Properly speaking, the original meaning of the word “light” is “something” that makes it possible to see, whereas in another, more analogical, sense, “light” can mean “something” that makes it possible for the other cognitive faculties to obtain the sort of knowledge that is proper to them. The first meaning goes back to the ambit of interaction with the senses, on a physical or material level; the second, on the other hand, analogy brings us into the immaterial ambit of knowledge. Thomas skillfully maintains our attention, for now, on the essential elements of the matter, avoiding getting into a physical theory of light which he deems unnecessary for this initial terminological clarification.

Click here for Thomas’s discussion, from

Click here for Thomas’s discussion, from a preview from Google Books of the Summa.