Full Idea
Thoughts are neither things in the external world nor ideas. A third realm must be recognised. Anything in this realm has it in common with ideas that it cannot be perceived by the senses, and does not need an owner to belong with his consciousness.
Gist of Idea
Thoughts in the 'third realm' cannot be sensed, and do not need an owner to exist
Source
Gottlob Frege (The Thought: a Logical Enquiry [1918], p.337(69))
Book Reference
Frege,Gottlob: 'The Frege Reader', ed/tr. Beaney,Michael [Blackwell 1997], p.337
A Reaction
This important idea is the creed for modern platonists. We don't have to accept Forms, or any particular content, but there is a mode of existence which is distinct from both mental and physical, and is the residence of 'abstracta'. I deny it!