Full Idea
Each tester of the truth of a theory …acts as if it were true, and expects the result to disappoint him if his assumption is false. The longer disappointment is delayed, the stronger grows his faith in his theory.
Gist of Idea
Experimenters assume the theory is true, and stick to it as long as result don't disappoint
Source
William James (The Sentiment of Rationality [1882], p.42)
Book Reference
James,William: 'Selected Writings of William James', ed/tr. Bird,Graham [Everyman 1995], p.42
A Reaction
This is almost exactly Popper's falsificationist proposal for science, which interestingly shows the close relationship of his view to pragmatism. Believe it as long as it is still working.