Part I. Question 1
Aristotle uses two methods to prove light cannot travel. Besides his empirical explanation, where he observes that for the supposed motion of light to go “unnoticed from where the sun rises to where it sets is asking too much” (418b26), he also provides an argument that is understood through the “light of reason.” (418b24) To understand his contention we must refer to his definitions of light and the transparent. The transparent is, for Aristotle, the medium of sight; it is “what is visible but not visible in its own right.”(418b5) Aristotle remarks that “air and water and many solid bodies are of this sort”(418b5), i.e. the sort of things we associate with transparency, but they are not the transparent as such, but rather share a common quality, which is the transparent. Light is not the visible, but that through which what is properly visible is seen. The transparent is only identifiable in virtue of its being-at-work-staying-the-same, light, without which the visible could not set into motion that which was being-at-work-staying-transparent. This being said, we can now understand Aristotle’s contention for the impossibility of light travel, which begins at 418b15. One way of understanding the argument is thus: Even if we assume that light does in fact travel, which runs counter to the empirical evidence of Aristotle’s epoch, this implies illumination ought to be a local motion of light passing through the transparent, now local motion is restricted to bodies, therefore if Aristotle can prove light is not a body the whole argument crumbles to the ground. To prove light is not a body Aristotle makes the following assumption, “two bodies could not be in the same thing at the same time.”(418b17) Thus if light was a body it would be incapable of co-existing with a transparent body as such. But light does co-exist with transparent bodies, this is proved by examining light and its contrary, where Aristotle states that “since darkness is the...
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