Full Idea
On Frege's view, what distinguishes thoughts from everything else is that they may meaningfully be called 'true' and 'false'.
Gist of Idea
A thought is distinguished from other things by a capacity to be true or false
Source
report of Gottlob Frege (The Thought: a Logical Enquiry [1918]) by Michael Dummett - Frege philosophy of mathematics Ch.2
Book Reference
Dummett,Michael: 'Frege: philosophy of mathematics' [Duckworth 1991], p.15
A Reaction
A lot of thinking is imagistic, and while the image may or may not truly picture the world, we tend to think that the truth or otherwise of daydreaming is simply irrelevant. Does Frege take all thought to be propositional?