Logic itself rests upon assumptions to which nothing in the world of reality corresponds. For example, the correspondence of certain things to one another and the identity of those things at different periods of time are assumptions pure and simple, but the science of logic originated in the positive belief that they were not assumptions at all but established facts. It is the same with the science of mathematics which certainly would never have come into existence if mankind had known from the beginning that in all nature there is no perfectly straight line, no true circle, no standard of measurement.