6.35 Although the spots in our picture are geometrical figures, geometry can still obviously say absolutely nothing about their actual form and position. But the net is purely geometrical, all its properties can be given a priori.
Laws, like the principle of sufficient reason, etc., deal with the net, not with what the net describes.
So geometry can be used to describe the spots, but there is no a priori knowing their shape or position. The net can be described a priori, though, so it belongs to logic. So what is the relation between a particular net and all possible nets?
Black (p. 361) says that “purely geometrical” is a reference to pure, as opposed to applied, geometry.