Thursday, 12 April 2018

Aristotle’s Theory of Light

In De Anima, Aristotle says that vision (or seeing) is the primary sense, and he models his analysis of the other senses on vision. But vision is not possible unless there is light, so Aristotle goes on to explain the nature of light.

In Aristotle, (Chapter 5, “The Power of Selective Response: Sensing and Knowing”), John Herman Randall notes that even though Aristotle didn’t have access to the kind of scientific and mathematical knowledge on light that we have today, his theory of light is so much like our own. Here’s an excerpt from Chapter 5 of Randall’s Aristotle:
Aristotle’s theory of color and light is quite a remarkable achievement. It runs: There is a transparent medium, to diaphanes, the Diaphanous, which is potentially light, and which becomes actual light when the sun or fire imparts motion to it. This motion of the transparent or diaphanous medium, when actualized as light, in turn actualizes the color of the wall—which is thus a kind of “second light”—in the seeing of the eye. Thus the answer to the question, What makes us see? What is the efficient cause of seeing? is that it is ultimately motion that makes us see, the motion imparted to the transparent medium by the light of illumination, and transmitted from the colored surface to the eye. For motion is the only agent, the only efficient cause, to be found in Aristotle: only motion can ever “make” things happen to him.  
Aristotle attempts to generalize from this example of seeing. The motion of some medium becomes for him the efficient cause of every kind of sensing: the motion of the transparent in seeing, of air or water in hearing, etc. He is generalizing from the distance receptors, and so he naturally gets into trouble when he comes to touch. What he comes out with is that “flesh” seems to be medium with that kind of sensing. The microscopic discovery of nerves would undoubtedly have delighted him. 
In other words, sensing for Aristotle is a “natural” or “physical” process, and not a “mental” one. He saw color, images, imagination, pain, pleasure, and all the passions and emotions as physical phenomena and not mental ones. According to Randall, it is doubtful if we have gone further than Aristotle in answering the question, What is light?

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