“The terrain is everything; the germ is nothing.”

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.

The true worth of an experimenter consists in his pursuing not only what he seeks in his experiment, but also what he did not seek.

The mental never influences the physical. It is always the physical that modifies the mental, and when we think that the mind is diseased, it is always an illusion.

Science increases our power in proportion as it lowers our pride.

When we meet a fact which contradicts a prevailing theory, we must accept the fact and abandon the theory, even when the theory is supported by great names and generally accepted.

The joy of discovery is certainly the liveliest that the mind of man can ever feel.

The experimenter who does not know what he is looking for will not understand what he finds.

If the ‘germ theory of disease’ were correct, there’d be no one living to believe it.