The subject of invariance or symmetry that I am going to talk about is an interesting subject for various reasons. To the philosopher of science it is particularly interesting because here he finds the physicist saying things that he usually does not say: he finds him making certain metastatements of a nonempirical status. Put a physicist before the work of Carnap and he will shrug his shoulders. He is more than happy, however, to state that the laws of electrodynamics are invariant under Lorentz transformations or to postulate the relativistic invariance of any future law. But in doing this the physicist does exactly the kind of thing that Carnap wanted a philosopher of science to do: he states or postulates something about a physical law. Moreover, his claims might even be of a purely syntactical nature: frequently one hears it said or reads it in print that it is the form of a law that is invariant. It seems, therefore, that invariance, if anything, is a subject of common interest to the philosopher and physicist, nicely suited to be dealt with in a meeting like this.