Full Idea
A sentence has truth conditions only in a context of use. And the truth conditions of many sentences can differ from one context of use to another (as in 'I am a philosopher').
Gist of Idea
A sentence's truth conditions depend on context
Source
Trenton Merricks (Propositions [2015], 1.II)
Book Reference
Merricks,Trenton: 'Propositions' [OUP 2015], p.6
A Reaction
He is building a defence of propositions, because they are eternal, and have their truth conditions essentially. I too am a fan of propositions.