Full Idea
I call a 'thought' something for which the question of truth can arise at all. ...So I can say: thoughts are senses of sentences, without wishing to assert that the sense of every sentence is a thought.
Gist of Idea
A 'thought' is something for which the question of truth can arise; thoughts are senses of sentences
Source
Gottlob Frege (The Thought: a Logical Enquiry [1918], p.327-8 (61))
Book Reference
Frege,Gottlob: 'The Frege Reader', ed/tr. Beaney,Michael [Blackwell 1997], p.328
A Reaction
This builds on his distinction between sense and reference. The reference of every truth sentence is just 'the true', and the sense is the proposition. The concept of a proposition seems indispensable to logic, I would say.