Full Idea
Truth is such a transcendentally clear notion that it cannot be further defined.
Gist of Idea
Truth is such a transcendentally clear notion that it cannot be further defined
Source
René Descartes (Letters to Mersenne [1640], 1642), quoted by Pascal Engel - Truth Intro
Book Reference
Engel,Pascal: 'Truth' [Acumen 2002], p.6
A Reaction
This is the view endorsed by Davidson. It is tempting to take basic concepts as axiomatic, but philosophers can't make that move every time they are in trouble. I have to say, though, that truth is a good candidate.