The stability of the internal medium is a primary condition for the freedom and independence of certain living bodies in relation to the environment … - Claude Bernard

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

Our language, in fact, is only approximate, and even in science it is so indefinite that if we lose sight of phenomena and cling to words, we are speedily outside of reality.… [W]e must always cling to phenomena and see in words only expressions empty of meaning, if the phenomena they represent are not definite, or if they are absent.

[I]t is important to determine at what point to apply doubt, so as to distinguish it from scepticism, and to show how scientific doubt becomes an element of the greatest certainty. The sceptic disbelieves in science and believes in himself; he believes enough in himself to dare deny science and to assert that it is not subject to definite, fixed laws. The doubter is a true man of science; he doubts only himself and his interpretations, but he believes in science; in the experimental sciences, he even accepts a criterion of absolute scientific principle.