In the first place a philosophical proposition must be general. It must not deal specially with things on the surface of the earth, or within the solar system, or with any other portion of space and time. . . . This brings us to a second characteristic of philosophical propositions, namely that they must be a priori. A philosophical proposition must be such as can neither be proved nor disproved by empirical evidence. . . . Philosophy, if what has been said is correct, becomes indistinguishable from logic as that word has now come to be used.

The resistance to a new idea increases by the square of its importance.

My own belief is that in most ages and in most places obscure psychological forces led men to adopt systems involving quite unnecessary cruelty, and that this is still the case among the most civilized races at the present day.

To be without some of the things you want is an indispensable part of happiness.

There can't be a practical reason for believing something that is not true.

The significance of a fact is relative to [the general body of scientific] knowledge. To say that a fact is significant in science, is to say that it helps to establish or refute some general law; for science, though it starts from observation of the particular, is not concerned essentially with the particular, but with the general. A fact, in science, is not a mere fact, but an instance. In this the scientist differs from the artist, who, if he deigns to notice facts at all, is likely to notice them in all their particularity.

Fools and fanatics are always so certain of themselves.