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I do not think the notion of rigidity in designation can be correct, at least not in any way that can serve to ground a semantics that purports both to be fundamental in a semiotical sense and to respect the best science of the day. A careful examination of both the content and the character of our best scientific knowledge not only cannot support anything like what the notion of rigidity requires, but actually shows the notion to be, at bottom, incoherent. In particular, the scientific meaning of natural kind terms can be determined only within the context of a fixed scientific framework and not sub specie æternitatis. Along the way, I provide grounds for the rejection of essentialist views o