Men are not allowed to think freely about chemistry and biology: why should they be allowed to think freely about political philosophy?

The word right should be excluded from political language, as the word cause from the language of philosophy.

Every science consists in the coordination of facts; if the different observations were entirely isolated, there would be no science.

Ideas govern the world, or throw it into chaos.

In mathematics we find the primitive source of rationality; and to mathematics must the biologists resort for means to carry out their researches.

All good intellects have repeated, since Bacon's time, that there can be no real knowledge but that which is based on observed facts. This is incontestable, in our present advanced stage; but, if we look back to the primitive stage of human knowledge, we shall see that it must have been otherwise then. If it is true that every theory must be based upon observed facts; it is equally true that facts can not be observed without the guidance of some theory. Without such guidance, our facts would be desultory and fruitless; we could not retain them: for the most part we could not even perceive them.

Religion is an illusion of childhood, outgrown under proper education.