  Instead of applying the grammatical criterion to artificial languages like FOL, one might apply it to natural languages like English. One could then appeal to the work of empirical linguists for a favored grammatical regimentation. Contemporary linguists posit a structural representation called LF that resolves issues of scope and binding crucial to semantic evaluation. But the question remains which lexical items in the LF should count as logical constants. Generalizing Quine’s proposal, one might identify the logical constants with members of small, “closed” lexical categories: for example, conjunctions and determiners. However, by this criterion prepositions in English would count as logical constants (Harman 1984, 121). Alternatively, one might identify the logical constants with members of functional categories (including tense, complementizers, auxiliaries, determiners, and pronouns), and the nonlogical constants with members of substantive categories (including nouns, verbs, adjectives, adverbs, and prepositions) (for this terminology, see Chomsky 1995, 6, 54 and Radford 2004, 41). If a distinction that plays an important role in a theory of linguistic competence should turn out to coincide (in large part) with our traditional distinction between logical and nonlogical constants, then this fact would stand in need of explanation. Why should we treat inferences that are truth-preserving in virtue of their LF structures and functional words differently from those that are truth-preserving in virtue of their LF structures and substantive words? Future work in linguistics, cognitive psychology and neurophysiology may provide the materials for an interesting answer to this question, but for now it is important that the question be asked, and that we keep in mind the possibility of a sceptical answer.