Among the symbolic languages used most frequently in the indirect interpretation of natural language are Montague's Intensional Logic IL [5, 384ff.] and its extensional counterpart, the language Ty2 of two-sorted type theory. The question of which of these two formal languages is to be preferred has been obscured by lack of knowledge about the exact relation between them. The present paper is an attempt to clarify the situation by showing that, modulo a small, decidable class of formulas irrelevant to these applications, IL and Ty2 are equivalent in the strong sense that there exists a reversible translation between the terms of either language.

In [3, 6Iff.] Gallin has shown that there exists a simple and natural translation * of IL into Ty2. Following Gallin's translation procedure, it is even possible to conceive of IL as a highly restricted sublanguage of Ty2, viz. as that part which only contains expressions of certain intensional types plus one variable of the basic type of indices or worlds. In an obvious sense, this sublanguage has less expressive power than the whole of Ty2, where it is possible to express conditions on entities that do not even exist in IL's ontology. However, by a certain amount of coding, one can translate Ty2 into IL [3, 105]. Conditions on nonintensional entities then become conditions on corresponding intensional objects; and these paraphrases preserve (standard) validity and entailment. On the other hand, this retranslation of Ty2 into IL is not an inversion of *, as can be seen from a simple example.