[Those] 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.

The great experimental principle, then, is doubt, that philosophic doubt which leaves to the mind its freedom and initiative, and from which the virtues most valuable to investigators in physiology and medicine are derived.

Experiment is fundamentally only induced observation.

Our ideas are only intellectual instruments which we use to break into phenomena; we must change them when they have served their purpose, as we change a blunt lancet that we have used long enough.

Put off your imagination, as you put off your overcoat, when you enter the laboratory. Put it on again, as you put on your overcoat, when you leave.

The investigator should have a robust faith - and yet not believe.

The doubter is a true man of science: he doubts only himself and his interpretations, but he believes in science.