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SOC. Let this, then, be one kind of motion. Now when a thing remains in the same place, but grows old, or becomes black instead of white, or hard instead of soft, or undergoes any other kind of alteration, is it not proper to say that this is another kind of motion?
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THEO. I think so.
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SOC. Nay, it must be true. So I say that there are these two kinds of motion: alteration, and motion in space.
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THEO. And you are right.
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SOC. Now that we have made this distinction, let us at once converse with those who say that all things are in motion, and let us ask them, Do you mean that everything moves in both ways, moving in space and undergoing alteration, or one thing in both ways and another in one of the two ways only?
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THEO. By Zeus, I cannot tell! But I think they would say that everything moves in both ways.
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SOC. Yes; otherwise, my friend, they will find that things in motion are also things at rest, and it will be no more correct to say that all things are in motion than that all things are at rest.
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THEO. What you say is very true.
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SOC. Then since they must be in motion, and since absence of motion must be impossible for anything, all things are always in all kinds of motion.
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THEO. Necessarily.