There has been a tendency in recent literature on personal identity to treat puzzle cases as unfair intrusions upon the discussion, like proposing to play chess without the Queen. Thus Terence Penelhum speaks of ‘imaginary worlds’ where our normal criteria do not hold and Sydney Shoemaker refers approvingly to G. C. Nerlich's dictum that it is a universal truth of our world, and not of ‘all possible worlds', that only by being identical with a witness to past events can one have the knowledge of them we have in memory.

I would agree that where puzzle cases involve changing basic features of our physical universe, e.g. in having people's bodies go out of existence in one place and reappear in another, as recently envisaged by J. M. Shorter, there is some point in talking of ‘imaginary’ or ‘possible’ worlds. But where puzzle cases propose no such basic changes but ground the discussion in physiological possibilities, however unfeasible technologically, this seems an arbitrarily harsh description. It would be like Nineteenth Century philosophers saying talk of flying machines and heart transplants belongs, not to another age merely, but to other worlds.