"It is what we know already that often prevents us from learning."
"Men who have excessive faith in their theories or ideas are not only ill prepared for making discoveries; they also make very poor observations. Of necessity, they observe with a preconceived idea, and when they devise an experiment, they can see, in its results,only a confirmation of their theory. In this way they distort observation and often neglect very important facts because they do not further their aim."
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Source: Introduction to the Study of Experimental Medicine (1865) Pt I, Ch. 2, Sect. III
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