I've just started reading Duhem for Philosophy of Science, and came across a rather beautiful quote (which, strangely, does not seem to be in any quote databases):
Men who have an excessive faith in their theories or in their ideas are not only poorly disposed to make discoveries but they also make very poor observations. They necessarily observe with a preconceived idea and, when they have begun an experiment, they want to see in its results only a confirmation of their theory. Thus they distort observation and often neglect very important facts because they go counter to their goal.
At least, I think the quote is from Duhem himself. It's in quotes in an article written by Duhem (Physical Theory and Experiment), but the only source cited is a book also written by Duhem. * shrugs * Whoever the real author is, I agree with the sentiment. "Excessive faith" might also be described as "attachment." A good scientist must remain detached from any particular result.
ADDENDUM: Google books has the article/chapter in question here. The section we're reading starts at "Section 1" preceding the place the link will go to.