Man does not limit himself to seeing; he thinks and insists on learning the meaning of phenomena whose existence has been revealed to him by observation. So he reasons, compares facts, puts questions to them, and by the answers which he extracts, tests one by another. This sort of control, by means of reasoning and facts, is what constitutes experiment, properly speaking; and it is the only process that we have for teaching ourselves about the nature of things outside us.

Real science exists, then, only from the moment when a phenomenon is accurately defined as to its nature and rigorously determined in relation to its material conditions, that is, when its law is known. Before that, we have only groping and empiricism.

Man can learn nothing unless he proceeds from the known to the unknown.

Art is 'I'; science is 'we'.

[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.

A fact in itself is nothing. It is valuable only for the idea attached to it, or for the proof which it furnishes.

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.