Underdetermination and the Problem of Identical Rivals

This paper explores a connection between underdetermination and the identity conditions for theories: Must we provide the latter before enquiring into the former? Quine proposes that we call two theories identical if they are equivalent under a reconstrual of predicates, but this may mishandle important cases. Another proposal is to defer to the particular judgements of actual scientists. Consideration of an historical episode--- the alleged equivalence of wave and matrix mechanics--- shows that this second proposal also fails. Nevertheless, I suggest, we may enquire into underdetermination without a better candidate