"Thought: A Logical Inquiry" is an essay by Gottlob Frege.
In it, Frege argues against idealism and for platonism about thoughts, or propositions.
[2][3] Frege says ideas are private, but thoughts are public.
[4] Frege also argued for a redundancy theory of truth.
Quoting Frege, "The thought, in itself immaterial, clothes itself in the material garment of a sentence and thereby becomes comprehensible to us.