Full Idea
Mathematics rigorously treated from the point of view of deducing theorems exclusively by means of introspective construction, is called intuitionistic mathematics. It deviates from classical mathematics, which believes in unknown truths.
Gist of Idea
Intuitionist mathematics deduces by introspective construction, and rejects unknown truths
Source
Luitzen E.J. Brouwer (Consciousness, Philosophy and Mathematics [1948]), quoted by Stewart Shapiro - Thinking About Mathematics 1.2
Book Reference
Shapiro,Stewart: 'Thinking About Mathematics' [OUP 2000], p.8
A Reaction
Clearly intuitionist mathematics is a close cousin of logical positivism and the verification principle. This view would be anathema to Frege, because it is psychological. Personally I believe in the existence of unknown truths, big time!