Invariance, symmetry and lawfulness

In this paper I attempt mainly to elucidate the claim, advanced by Woorward (1992), that the key notion to characterize physical laws is that of invariance. I draw a distinction betwen two levels of invariance in order to elaborate that thesis. I maintain that distinctive marks of the nomic status of basic laws of physics are either that they hold invariantly, within a domain of applivation, or that they fulfill some principles of symmetry. The fomer mark relatesd to the manner in which physical systems change invariantly whereas the latter concerns to the invariance of the laws themselves. This view contrats with the traditional philosophical thesis that physical laws are true universal statements with a necessary character which differentiate then from accidental true generalizations, which are contingent.