[1051a] [1] for they will be in a truer sense actualities, whereas knowledge and motion will be their potentialities.1 Thus it is obvious that actuality is prior both to potentiality and to every principle of change.

That a good actuality is both better and more estimable than a good potentiality will be obvious from the following arguments. Everything of which we speak as capable is alike capable of contrary results; e.g., that which we call capable of being well is alike capable of being ill, and has both potentialities at once; for the same potentiality admits of health and disease, or of rest and motion, or of building and of pulling down, or of being built and of falling down.Thus the capacity for two contraries can belong to a thing at the same time, but the contraries cannot belong at the same time; i.e., the actualities, e.g. health and disease, cannot belong to a thing at the same time. Therefore one of them must be the good; but the potentiality may equally well be both or neither. Therefore the actuality is better.

Also in the case of evils the end or actuality must be worse than the potentiality; for that which is capable is capable alike of both contraries.

Clearly, then, evil does not exist apart from things ; for evil is by nature posterior to potentiality.2 [20] Nor is there in things which are original and eternal any evil or error, or anything which has been destroyedâ€”for destruction is an evil.

Geometrical constructions, too, are discovered by an actualization, because it is by dividing that we discover them. If the division were already done, they would be obvious; but as it is the division is only there potentially. Why is the sum of the interior angles of a triangle equal to two right angles? Because the angles about one point <in a straight line> are equal to two right angles. If the line parallel to the side had been already drawn, the answer would have been obvious at sight.3Why is the angle in a semicircle always a right angle? If three lines are equal, the two forming the base, and the one set upright from the middle of the base, the answer is obvious to one who knows the former proposition.4 Thus it is evident that the potential constructions are discovered by being actualized. The reason for this is that the actualization is an act of thinking. Thus potentiality comes from actuality (and therefore it is by constructive action that we acquire knowledge). <But this is true only in the abstract>, for the individual actuality is posterior in generation to its potentiality.5

The terms "being" and "not-being" are used not only with reference to the types of predication, and to the potentiality or actuality, or non-potentiality and non-actuality, of these types,

1 This is a passing thrust at the Ideal theory. "Absolute knowledge" (the faculty of knowledge) will be a mere potentiality, and therefore substantially posterior to its actualization in particular instances.

2 The argument is presumably as follows (the fallacy, as pointed out by Bonitz, is indicated in parenthesis): That which has a separate substantial existence is actuality. Actuality is prior (substantially) to potentiality. Potentiality is prior to evil (in the moral scale. But since by evil Aristotle means the actualization of a potentiality for evil, potentiality is substantially posterior to evil). Therefore that which has a separate substantial existence is prior to evil; i.e., evil does not exist apart from particular instances of evil. The argument is directed against the Platonic Idea of evil (Plat. Rep. 476a); and the corollary which follows against the identification of Evil with one of the principles of the universe (Aristot. Met. 1.6.10, Aristot. Met. 12.10.6, Aristot. Met. 14.4.10, 11; cf. Plat. Laws 896e, Plat. Laws 898c).

3 The figure, construction and proof are as follows: ***

4 Aristotle implies a proof something after this fashion: FIGURE BAC is an angle in a semicircle. From D, the mid-point of the diameter BC, draw a perpendicular DE to meet the circumference at E. Join EB, EC.***

5 This whole passage (sects. 4, 5) should be compared with Aristot. Met. 9.8.3-7, where it logically belongs.