“In the philosophic sense, observation shows and experiment teaches.”

In science, the best precept is to alter and exchange our ideas as fast as science moves ahead.

We must remain, in a word, in an intellectual disposition which seems paradoxical, but which, in my opinion, represents the true mind of the investigator. We must have a robust faith and yet not believe.

True science teaches us to doubt and, in ignorance, to refrain.

Those who do not know the torment of the unknown cannot have the joy of discovery.

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

Obervation is a passive science, experimentation is an active science.

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

The fact that knowledge endlessly recedes as the investigator is about to grasp it is what constitutes at the same time his torment and happiness.

If I had to define life in a single phrase, I should clearly express my thought of throwing into relief one characteristic which, in my opinion, sharply differentiates biological science. I should say: life is creation.

First causes are outside the realm of science.

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.

A great discovery is a fact whose appearance in science gives rise to shining ideas, whose light dispels many obscurities and shows us new paths.

A discovery is generally an unforeseen relation not included in theory.

Well-observed facts, though brought to light by passing theories, will never die; they are the material on which alone the house of science will at last be built.

Particular facts are never scientific; only generalization can establish science.

Men who believe too firmly in their theories, do not believe enough in the theories of others. So these despisers of their fellows make experiments only to destroy a theory, instead of to seek the truth.