Intuitively, two expressions are synonymous if and only if (iff) they have the same meaning. Despite the apparently straightforward nature of this definition, the notion of synonymy has been hard pressed in contemporary philosophy of language. Difficulties arise from two directions: general skepticism about intensional semantics and specific concerns involving substitution into intensional contexts.

Quine Against Intensional Semantics

In "Two Dogmas of Empiricism" (1951), W. V. O. Quine leveled an influential attack on intensional semantic concepts, concepts that express meaning relations (e.g., "analytic," "synonymous," and "antonymous"). While Quine acknowledged that such concepts are as a group interdefinable, he argued that no members of the class can be made philosophically respectable on empiricist principles.

His argument can be stated as follows. To be acceptable, semantic concepts must be definable in terms that are either formal (i.e., purely logical or linguistic) or observational. Quine first argued that...