Sir Harold Jeffreys (1958) has asserted that my analysis of the clock paradox (Builder 1957) is based on concealed hypotheses; but the hypotheses to which he refers are ones that he claims are involved in the derivation of the Lorentz transformations and in the currently accepted methods of using them. He holds that the transformations cannot be shown to be unique and he infers that predictions based on them are not necessarily correct and " cannot be right ill any case" when applied to systems that are inertial at the time considered but that have been subject to acceleration at some other time.