As indicated in the Introduction, the aim of Philosophy of Science ought to be a scientific study of the methods scientists use. Although finished bodies of knowledge appear to be elegant, the arsenal of working hypotheses looks chaotic. There are rules of thumb applicable within certain domains and there are quaint violations of the rules in certain others and so on. In organising these into a theory, scientists rely on intuition and on tentative ground-rules which are themselves external to scientific knowledge. In this sense the ground-rules are metaphysical. The symmetry paradigm described above is one such ground-rule, matured enough to be called a metaphysical principle. It is well articulated and actively pursued. It is hoped that it proves to be of some interest in the study of Philosophy.