Book 2
4
But the strict sense of first principle is not found in first principles incapable of movement, for example those of mathematics, although the term is indeed used of them by analogy, for in mathematics if the first principle were changed virtually all the things proved from it would change, though they do not change owing to themselves, one being destroyed by the other, except by destroying the assumption and thereby establishing a proof.e.g. if ἀρχή A led to B and C, of which C was absurd, then C by refuting A would refute the other consequence B (Solomon).
5
But man is a first principle of a certain motion, for action is motion. And since as in other matters the first principle is a cause of the things that exist or come into existence because of it, we must think as we do in the case of demonstrations.
6
For example, if as the angles of a triangle are together equal to two right angles the angles of a quadrilateral are necessarily equal to four right angles, that the angles of a triangle are equal to two right angles is clearly the cause of that fact; and supposing a triangle were to change, a quadrilateral would necessarily change too—for example if the angles of a triangle became equal to three right angles, the angles of a quadrilateral would become equal to six right angles, or if four, eight; also if a triangle does not change but is as described, a quadrilateral too must of necessity be as described.
7
The necessity of what we are arguing is clear from AnalyticsCf. Aristot. Anal. Post. 1.1; at present we cannot either deny or affirm anything definitely except just this. Supposing there were no further cause of the triangle’s having the property stated, then the triangle would be a sort of first principle or cause of the later stages.
