Verbal Behavior: Extended Edition. Chapter 1: A Functional Analysis of Verbal Behavior. Quote 23

To define a proposition as “something which may be said in any language” does not tell us where propositions are, or of what stuff they are made. Nor is the problem solved by defining a proposition as all the sentences which have the same meaning as some one sentence, since we cannot identify a sentence as a member of this class without knowing its meaning—at which point we find ourselves facing our original problem. (p. 8)