This is why we can say, if we want, that the proof consists in bringing the sum of the angles constructed into two different groupings, and seeing that sum alternately as equal to the sum of the angles of the triangle, and equal to two right angles, but it must be add that here we have not merely two successive configurations, the first of which eliminates the second (as is the case with the child sketching dreamily); the first survives for me while the second is in process of establishing itself, the sum of angles which I equate with two right angles is the same as I elsewhere equate with the sum of the angles of the triangle, all of which is possible