Full Idea
To understand a proposition means to know what is the case if it is true.
Gist of Idea
To understand a proposition means to know what is the case if it is true
Source
Ludwig Wittgenstein (Tractatus Logico-Philosophicus [1921], 4.024)
Book Reference
Wittgenstein,Ludwig: 'Tractatus Logico-Philosophicus (Pears)', ed/tr. Pears,D. /McGuinness,B. [RKP 1961], p.21
A Reaction
This established the Frege truth-conditions theory of meaning, which was expanded by Davidson, and then possible worlds semantics. You can't assess truth without knowing meaning. Dummett says the two go together.