In 1954 Hempel wrote “Once the idea of a partial specification of meaning is granted, it appears unnecessarily restrictive, however, to limit the sentences effecting such partial interpretation to reduction sentences in Carnap's sense. … Generally, then, a set of one or more theoretical terms, t1, t2 ⃛, tn, might be introduced by any set M of sentences such that M contains no extralogical terms other than t1, t2 ⃛, tn, and observation terms, M is logically consistent, and M is (...) not equivalent to a truth of formal logic. The last two of these conditions serve merely to exclude trivial extreme cases. A set M of this kind will be referred to briefly as an interpretative system, its elements as interpretative sentences.” This is the last move in a process in which the idea of definition has been widened beyond the explicit or equivalence definition which was the basis of earlier thought on the subject of introducing new words into a language. It would seem that the scheme proposed by Hempel must represent the final move, for it would seem difficult to conceive any more liberal description of a purely direct and verbal scheme. Among the predecessors of Hempel in this liberalising movement I will mention Ramsey and Braithwaite whose views stem ultimately from Campbell, and are based on the idea that the “interpretative system” is essentially a scientific theory, and Carnap who introduced the particular notion of a “reduction sentence”, and who was more directly the source of Hempel's own ideas. (shrink)