 According to the structuralist picture (Sneed 1971; further developed in: Stegmüller 1979), to give an account of a theory T is to define the predicate ‘_is a (model of) T’ in set-theoretical terms. A theory basically consists of a set M, a set of potential models MP, a set of potential partial models MPP and a set of intended applications I, which is supposed to be “open”, or non-extensionally individuated. The relation between the different variants or stages of I is, as Stegmüller suggests, best thought of as being pragmatically fixed (Stegmüller 1979). Building on such a notion of a theory, Patrick Suppes proposes the following characterization of inter-theory reduction: