 As regards the credentials of Stewart’s ‘incontrovertible logical maxim’, if the claim that human nature is invariant is an empirical claim, it must be based on observation of our contemporaries and on evidence of people’s lives in other places and at other times. Such evidence needs however to be handled with care. The further back we go the more meagre it is, and so the more we need to conjecture to supplement the few general facts available to us. Indeed we can go back so far that we have no facts beyond the generalities that we have worked out in the light of our experience. But to rely on conjecture in order to support the very principle that forms the first premiss in any exercise in conjectural history is to come suspiciously close to arguing in a circle. The incontrovertible logical maxim of Dugald Stewart should probably be accorded at most the status of a well-supported empirical generalisation.