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.

Tout est poison, rien n'est poison, tout est une question de dose. Everything is poisonous, nothing is poisonous, it is all a matter of dose.

We achieve more than we know. We know more than we understand. We understand more than we can explain.

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

Mediocre men often have the most acquired knowledge

In the philosophic sense, observation shows and experiment teaches.

Theories are like a stairway; by climbing, science widens its horizon more and more, because theories embody and necessarily include proportionately more facts as they advance.