I have heard that Epicurus stated that light has the speed of thought. What did he mean by this?
My hypothesis is he intended to say, in a way, that the speed of light is infinite. But then, why was he so figurative? Is there some hidden meaning?
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I've found in Norman Wentworth DeWitt, Epicurus and His Philosophy (1954), page 163 :
Lacking a unit for the denotation of extremely high velocity, Epicurus describes it as follows:
"Furthermore, motion through the void, so long as no interference arises from conflicting bodies, accomplishes any conceivable distance in a space of time inconceivably brief." [Diogenes Laertius, Lives of the Ancient Philosophers, translation by R.D.Hicks, Loeb Classical Library, 1925, 10.2]
Elsewhere he also writes:
"For over whatever distance it [an object] holds to either motion [for every motion he assumes there is a motion in an opposite direction], over that distance it will maintain its speed, quick as thought." [Cicero, De Natura Deorum, 1.26.72]
What he meant by "quick as thought" may readily be surmised. He must have observed, first of all, the extraordinary speed of reaction in the automatic mind, which guards the safety of the individual during his daily rounds. To the ancients the chief hazards seem to have been carts, dogs, ditches, and precipices. We should think rather of the perils of motorized traffic.
I think that the statement must be interpreted as metaphorical ...
See also, in Epicurus, The extant remains (Cyril Bailey ed - 1926) : Epicurus to Herodotus, page 36 :
the atoms must move with equal speed, when they are borne onward through the void. [...] For as long as either of the two motions [upwards and downwards] prevails, so long will it have a course as quick as thought [...]. Moreover, their passage through the void, when it takes place without meeting any bodies which might collide, accomplishes every compehensible distance in an inconceivably short time.
Please, note that Epicurus is speaking of motion of atoms in general, and not specifically of light...