Full Idea
A reasonable way of explaining an expression is by saying what conditions make its various contexts true.
Gist of Idea
A good way of explaining an expression is saying what conditions make its contexts true
Source
Willard Quine (Philosophy of Logic [1970], Ch.3)
Book Reference
Quine,Willard: 'Philosophy of Logic' [Prentice-Hall 1970], p.40
A Reaction
I like the circumspect phrasing of this, which carefully avoids any entities such as 'meanings' or 'truth conditions'. Maybe the whole core of philosophy of language should shift from theories of meaning to just trying to 'explain' sentences.