Full Idea
The sentence 'I smell the scent of violets' has just the same content as 'It is true that I smell the scent of violets'. So it seems that nothing is added to the thought by my ascribing to it the property of truth.
Gist of Idea
The property of truth in 'It is true that I smell violets' adds nothing to 'I smell violets'
Source
Gottlob Frege (The Thought: a Logical Enquiry [1918], p.328 (61))
Book Reference
Frege,Gottlob: 'The Frege Reader', ed/tr. Beaney,Michael [Blackwell 1997], p.328
A Reaction
This idea predates Ramsey's similar proposal, for which, oddly, Ramsey always seems to get the credit. To a logician they may have identical content, but pragmatically they are likely to differ in context. 'True' certainly doesn't add to the thought.