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

But all of my efforts served only to make me better acquainted with the difficulty, which in itself was something.

. . . by natural selection our mind has adapted itself to the conditions of the external world. It has adopted the geometry most advantageous to the species or, in other words, the most convenient. Geometry is not true, it is advantageous.

One would have to have completely forgotten the history of science so as to not remember that the desire to know nature has had the most constant and the happiest influence on the development of mathematics.

It is far better to foresee even without certainty than not to foresee at all.

It is the simple hypotheses of which one must be most wary; because these are the ones that have the most chances of passing unnoticed.

It may be appropriate to quote a statement of Poincare, who said (partly in jest no doubt) that there must be something mysterious about the normal law since mathematicians think it is a law of nature whereas physicists are convinced that it is a mathematical theorem.