"Whoever believes that the only important reality is the realm of ideas, of the spirit, should not occupy himself with science. The scientist must be a realist, he must accept his sense impressions as more than hallucinations, as messages of a real outer world. In disentangling these messages he uses ideas of a very abstract kind, group theory in spaces of many or even infinitely many dimensions and things like that, but finally he has his observational invariants representing real things with which he learns to operate like any craftsman with his wood or metal."