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On Sense And The Sensible   

part, is manifestly smooth. The phenomenon of the flash occurs only

when the eye is moved, because only then could it possibly occur

that the same one object should become as it were two. The rapidity of

the movement has the effect of making that which sees and that which

is seen seem different from one another. Hence the phenomenon does not

occur unless the motion is rapid and takes place in darkness. For it

is in the dark that that which is smooth, e.g. the heads of certain

fishes, and the sepia of the cuttle-fish, naturally shines, and,

when the movement of the eye is slow, it is impossible that that which

sees and that which is seen should appear to be simultaneously two and

one. But, in fact, the eye sees itself in the above phenomenon

merely as it does so in ordinary optical reflexion.

If the visual organ proper really were fire, which is the doctrine

of Empedocles, a doctrine taught also in the Timaeus, and if vision

were the result of light issuing from the eye as from a lantern, why

should the eye not have had the power of seeing even in the dark? It

is totally idle to say, as the Timaeus does, that the visual ray

coming forth in the darkness is quenched. What is the meaning of

this 'quenching' of light? That which, like a fire of coals or an

ordinary flame, is hot and dry is, indeed, quenched by the moist or

cold; but heat and dryness are evidently not attributes of light. Or

if they are attributes of it, but belong to it in a degree so slight

as to be imperceptible to us, we should have expected that in the

daytime the light of the sun should be quenched when rain falls, and

that darkness should prevail in frosty weather. Flame, for example,

and ignited bodies are subject to such extinction, but experience

shows that nothing of this sort happens to the sunlight.

Empedocles at times seems to hold that vision is to be explained

as above stated by light issuing forth from the eye, e.g. in the

following passage:-

As when one who purposes going abroad prepares a lantern,

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