The terrain is everything; the germ is nothing.

We must never make experiments to confirm our ideas, but simply to control them.

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.

The stability of the internal medium is a primary condition for the freedom and independence of certain living bodies in relation to the environment surrounding them.

It is what we know already that often prevents us from learning.

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.

We must keep our freedom of mind, ... and must believe that in nature what is absurd, according to our theories, is not always impossible.