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

Mediocre men often have the most acquired knowledge

It is impossible to devise an experiment without a preconceived idea; devising an experiment, we said, is putting a question; we never conceive a question without an idea which invites an answer. I consider it, therefore, an absolute principle that experiments must always be devised in view of a preconceived idea, no matter if the idea be not very clear nor very well defined.

First causes are outside the realm of science.

[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.

But while I accept specialization in the practice, I reject it utterly in the theory of science.

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