If I have two words with different meaning, but which stand to each other in the relation of logical equivalence, do they form a tautology when one predicates the other? For example, if “the fittest” is both sufficient and necessary condition for “survives”, is the “the fittest survives” statement a tautology then? Also, is there a formal definition for synonyms? Do they have to be logically equivalent but different in meaning? Do they have to imply one another, be the same in meaning, but different logical relation? Thanks, Pavel.