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SOC. Perception, then, is always of that which exists and, since it is knowledge, cannot be false.
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THEAET. So it seems.
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SOC. By the Graces! I wonder if Protagoras, who was a very wise man, did not utter this dark saying to the common herd like ourselves, and tell the truth An allusion to the title of Protagoras’s book, Truth. in secret to his pupils.
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THEAET. Why, Socrates, what do you mean by that?
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SOC. I will tell you and it is not a bad description, either, that nothing is one and invariable, and you could not rightly ascribe any quality whatsoever to anything, but if you call it large it will also appear to be small, and light if you call it heavy, and everything else in the same way, since nothing whatever is one, either a particular thing or of a particular quality; but it is out of movement and motion and mixture with one another that all those things become which we wrongly say are—wrongly, because nothing ever is, but is always becoming. And on this subject all the philosophers, except Parmenides, may be marshalled in one line—Protagoras and Heracleitus and Empedocles—and the chief poets in the two kinds of poetry, Epicharmus, in comedy, and in tragedy, Homer, who, in the line0ceanus the origin of the gods, and Tethys their mother,Hom. Il. 14.201, 302has said that all things are the offspring of flow and motion; or don’t you think he means that?
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THEAET. I think he does.
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SOC. Then who could still contend with such a great host, led by Homer as general, and not make himself ridiculous?
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THEAET. It is not easy, Socrates.
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SOC. No, Theaetetus, it is not. For the doctrine is amply proved by this, namely, that motion is the cause of that which passes for existence, that is, of becoming, whereas rest is the cause of non-existence and destruction; for warmth or fire, which, you know, is the parent and preserver of all other things, is itself the offspring of movement and friction, and these two are forms of motion. Or are not these the source of fire?
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THEAET. Yes, they are.