G. L. Piggott’s paper “Implications of linguistic change for concrete phonology” does not present diachronic evidence in favor of a theory of phonology with abstract representations and derivations, as it claims, but rather reconstructs a hypothetical sequence of events. This reconstruction is based on a theory of phonology; that is, Piggott reconstructs a sequence of events that is consistent with the theory that he holds. It is not legitimate, therefore, to turn around and use this reconstructed situation as an argument for his theory. We return to this point below.
A somewhat weaker point is in principle possible, and Piggott tries to make it also. This would be to show that some theory cannot provide an historical account of how a certain situation could have arisen. Note that all that is required here is that the theory provide sufficient machinery to produce a plausible reconstruction of a state of affairs. Piggott is quite correct in maintaining that this is a legitimate test of a theory. He argues that the theory he defends provides such an account of the examples he discusses, but that the theory of Hooper (1976), natural generative phonology, cannot provide such an account.