You are urgently warned against allowing yourself to be influenced in any way by theories or by other preconceived notions in the observation of phenomena, the performance of analyses and other determinations.

Laboratory Rules at Munich. Quoted by M. Bergmann, 'Fischer', in Bugge's Das Buch der Grosse Chemiker. Trans. Joseph S. Fruton, Contrasts in Scientific Style: Research Groups in the Chemical and Biomedical Sciences (1990)