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2 The Physics of Light When Thomas Aquinas, or any of his medieval contemporaries, writes about light, we often import contemporary understandings of light into their discussions, but the fact is that Aquinas understood the nature of light very differently than we do. He completely rejected two of our understandings of light, since he believed that light could not travel as fast as it does and that it could not travel through a vacuum. When Aquinas writes about light, if we think that he understands it the same way we do we are at serious risk of misunderstanding him. The purpose of this chapter, then, is to describe in detail Aquinas’s understanding of light, so that when we turn to his theological application of light we will be able to understand him on his terms, without the anachronistic imposition of modern understandings of light. Additionally, by understanding how Aquinas thinks about light, we can observe how he uses his knowledge of the natural world to expand his understanding of God, since he believes that we can come to know some truths about God from God’s created effects. The study of physical light is important in understanding Aquinas’s theology of light because he makes a direct connection between physical light and spiritual light. For Aquinas it is fruitful to understand physical light as a means of understanding spiritual and theological light: Sense perceptible light, however, is a certain image of spiritual light, for every sensible thing is something particular, whereas intellectual things are a kind of whole. Just as particular light has an effect on the thing seen, inasmuch as it makes colors actually visible, as well as on the one seeing, because through it the eye is conditioned for seeing, so intellectual light makes the intellect to know because whatever light is in the rational creature is all derived from that supreme light “which illumines every man coming into the world.” Furthermore, it makes all things to be actually intelligible inasmuch as all forms 47 are derived from it, forms which give things the capability of being known, just as all the forms of artifacts are derived from the art and reason of the artisan.1 To see how Aquinas thinks light works we will look primarily at three texts, De Anima II.14–15, De Sensu 2–6, and Summa Theologiae I 67. Additionally, we will look at the role that light plays in vision, because Aquinas’s discussions of light invariably are intertwined with discussions of vision and because our final end is the vision of God, which requires the light of glory. As David Lindberg notes, “Before 1600 the science of optics tended to coalesce around two interrelated, yet distinguishable, problems—the nature and propagation of light, and the process of visual perception.”2 Light and vision go together, both from a scientific perspective, and in the case of Aquinas, from a theological perspective. At the outset, let me point out that among medieval theologians there is nothing especially novel about Aquinas’s understanding of light. Unlike his teacher, Albert the Great, Aquinas, while not ignorant of the natural world, generally kept his constructive, noncommentary work focused on theology. His commentary on Aristotle’s De Anima is dependent upon Albert’s work as well as that of Themistius and his commentary on De Sensu is reliant upon a commentary by Alexander of Aphrodisias.3 This is not to say that there was one shared concept of light and vision among medieval theologians, or that others did not consider Aquinas’s thoughts on light of interest. His treatment of light in De Anima II.14 was broken out and published as a separate tract, De Natura Luminis, which had its own manuscript history, so his contemporaries did think that he had something important to say about light.4 Additionally, there were alternative understandings of light and vision that were live options in the thirteenth century, particularly the theory of extramission that came from the 1. In Ioh. 8.2 §1142. 2. David C. Lindberg, Theories of Vision from al-Kindi to Kepler (Chicago: University of Chicago Press, 1981), x. 3. On the reliance upon Albert and Themestius, see Robert Pasnau’s introduction in: Thomas Aquinas, A Commentary on Aristotle’s De Anima, trans. Robert Pasnau, Yale Library of Medieval Philosophy (New Haven: Yale University Press, 1999), xiv. On the reliance upon Alexander, see ———, Commentaries on Aristotle’s “On Sense and What Is Sensed” and “On...

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