The advance of science is not comparable to the changes of a city, where old edifices are pitilessly torn down to give place to new, but to the continuous evolution of zoologic types which develop ceaselessly and end by becoming unrecognisable to the common sight, but where an expert eye finds always traces of the prior work of the centuries past. One must not think then that the old-fashioned theories have been sterile and vain.

Intuition is more important to discovery than logic.

Most striking at first is the appearance of sudden illumination, a manifest sign of long unconscious prior work.

It may happen that small differences in the initial conditions produce very great ones in the final phenomena.

Experiment is the sole source of truth. It alone can teach us something new; it alone can give us certainty.

Hypotheses are what we lack the least.

Sociology is the science with the greatest number of methods and the least results.