Full Idea
Not to the mathematician, but to the psychologist, belongs the task of explaining why ...we are averse to so-called contradictory systems in which the negative as well as the positive of certain propositions are valid.
Gist of Idea
Our dislike of contradiction in logic is a matter of psychology, not mathematics
Source
Luitzen E.J. Brouwer (Intuitionism and Formalism [1912], p.79)
Book Reference
'Philosophy of Mathematics: readings (2nd)', ed/tr. Benacerraf/Putnam [CUP 1983], p.79
A Reaction
Was the turning point of Graham Priest's life the day he read this sentence? I don't agree. I take the principle of non-contradiction to be a highly generalised observation of how the world works (and Russell agrees with me).