Full Idea
If sentences can have truth-values only when they occur as asserted, it would be impossible to have a truth-functional basis to logic.
Gist of Idea
Logic seems to work for unasserted sentences
Source
D.J. O'Connor (The Correspondence Theory of Truth [1975], Ch.6)
Book Reference
O'Connor,D.J.: 'The Correspondence Theory of Truth' [Hutchinson 1975], p.40