Application in science has its own structure, distinct from the structure of theoretical science, and therefore needs its own philosophy. The covering power of a formal scientific theory is no guide to its explanatory power. Explanation is too much to ask of a fundamental scientific theory. This is seen by considering two strands of the Born-Einstein debate: first the explanatory power of quantum mechanics and second, the reality of unobserved properties. The function of theoretical physics is to describe rather than to explain. Some techniques are a standard part of theory; while some aread hoc to the problems at hand. Very few of the derivations in mathematical physics are explanatory. This shows distinctly separate structures for theory and for application.