Wednesday, July 15, 2009

For Aquinas the eye is not a source of light

"the illumination of the material sun does not bring about in the bodily eye some light connatural to it, through which it can make things actually visible [(lumen aliquod sibi connaturale, per quod possit facere visibilia in actu)], as does happen in our mind by the illustration of the uncreated sun. Therefore the eye always needs an exterior light, but mind does not."

Thomas Aquinas, Expositio super librum Boethii De Trinitate, pars 1 q. 1 a. 1 ad 3, as translated by Ralph McInerny (Thomas Aquinas: selected writings, ed. and trans. with an introduction and notes by Ralph McInerny (London: Penguin Books, 1998), 114). The Latin is taken from the Leiden (i.e. Brill) edition of 1959, ed. Bruno Decker, as reproduced in Corpus Thomisticum here: At the beginning of the Responsio Aquinas stresses "the difference between active and passive powers", and calls even some lower powers (the vegetative) active, but not the sensitive: because "the passive powers cannot actually perform their proper operations unless moved by the active", "sense cannot sense unless it is moved by the sensible", and "the bodily eye can see bodies only by means of the supervenient illumination of the material sun" (arg. 3,, which isn't contradicted by ad 3). Clearly, then, the eye is not for Aquinas illuminative; it is not a source of light. I don't know that anyone has ever claimed this for Aquinas specifically, and I haven't read any of the specialized histories of ancient or medieval optics, but I have, I think, heard this claim made for the pre-moderns generally. Cf. this: Although "Archytas of Tarentum . . . held that vision arises as the effect of an invisible 'fire' emitted from the eyes so that on encountering objects it may reveal their shapes and colors", "the possible solutions of the problem of vision were [in antiquity] very limited; either something from the object arrives at the eye, or something from the eye goes out to the object, or else the intervening medium serves as the connection between the object and the eye" (Vasco Ronchi, "Optics and vision," Dictionary of the history of ideas, ed. Philip P. Wiener (New York, NY: Charles Scribner's Sons, 1973), vol. 3, p. 407; cf.

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