"Let us... see the geometer at work and seek to catch his process. The task is not without difficulty; it does not suffice to open a work at random and analyze any demonstration in it. We must first exclude geometry, where the question is complicated by arduous problems relative to the rôle of the postulates, to the nature and the origin of the notion of space. For analogous reasons we can not turn to the infinitesimal analysis. We must seek mathematical thought where it has remained pure, that is in arithmetic."