Nor is the said beginning an arbitrary and only temporary assumption, or something which seems to be an arbitrary and tentative presupposition but of which it is subsequently shown that to make it the starting point was indeed the right thing to do; this is not as when we are instructed to make certain constructions in order to aid the proof of a geometrical theorem, and only in retrospect, in the course of the proof, does it become apparent that we did well to draw precisely these lines