…the principle […] called pragmatism, namely that an intellectual concept is nothing but a concept of a purpose that might be entertained under conceivable circumstances.
This quote has been taken from Kenneth Laine Ketner's 1983 reconstruction of Peirce's 'Autobiography'. Ketner identifies the source as "variant pages" of the manuscript.
http://www.commens.org/dictionary/entry/quote-brief-intellectual-autobiography-charles-sanders-peirce-21