Men who have excessive faith in their theories ... make poor observations, because they choose among the results of their experiments only what suits their object, neglecting whatever is unrelated to it and carefully setting aside everything which might tend toward the idea they wish to combat

[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.

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.

Science does not permit exceptions.

Laplace considers astronomy a science of observation, because we can only observe the movements of the planets; we cannot reach them, indeed, to alter their course and to experiment with them. "On earth," said Laplace, "we make phenomena vary by experiments; in the sky, we carefully define all the phenomena presented to us by celestial motion." Certain physicians call medicine a science of observations, because they wrongly think that experimentation is inapplicable to it.

The goal of scientific physicians in their own science ... is to reduce the indeterminate. Statistics therefore apply only to cases in which the cause of the facts observed is still indeterminate.

The minds that rise and become really great are never self-satisfied, but still continue to strive.