Book 11
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That this account is correct is clear from what the other authorities say about motion, and from the fact that it is not easy to define it otherwise. For one thing, it could not be placed in any other class; this is clear from the fact that some peoplePythagoreans and Platonists. Cf. Aristot. Met. 1.5.6, Plat. Soph. 256d. identify it with otherness and inequality and not-being, none of which is necessarily moved;
9
moreover change is no more into these or out of them than into or out of their opposites.The criticism implied is: If motion is identified with otherness, inequality, etc., then these concepts must be either (a) subjects of motion, which is absurd, or (b) termini of motion, in which case the same must be true of their contraries, since motion is between contraries. The reason for placing motion in this class is that it is considered to be indeterminate, and the principles in one of the columns of contraries are indeterminate, being privative; for none of them is a determinate thing or quality or any of the other categories.
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The reason for considering motion to be indeterminate is that it cannot be associated either with the potentiality or with the actuality of things; for neither that which is potentially nor that which is actually of a certain size is necessarily moved.
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And motion is considered to be a kind of actualization, but incompleteCf. note on sect. 2 (end) above, and Aristot. Met. 9.6.7-10.; the reason of this is that the potential, of which it is the actualization, is incomplete.
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Thus it is difficult to comprehend what motion is; for we must associate it either with privation or with potentiality or with absolute actuality; and apparently none of these is possible.
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There remains, then, the account which we have given; that it is an actuality, and an actuality of the kind which we have described, which is hard to visualize but capable of existing.
