One restrictive position holds that substitution might be allowed at the level of individual words, but not for complete composite expressions. On this view, replacing a particular word with a genuine equivalent is not the same operation as restructuring an entire statement.
A composed utterance conveys negation, emphasis, restriction, conditions, and order through relationships among its words. Changing those relationships can change the sense even when each individual word appears familiar.
Illustration: “Only at designated times” differs from “at designated times.” Omitting “only” removes an exclusive restriction. The example demonstrates why linguistic fidelity requires attention to the meaning of the whole construction, not just synonyms in isolation.