Experience without theory is blind, but theory without experience is mere intellectual play.

Reason should investigate its own parameters before declaring its omniscience.

Enlightenment is man's emergence from his self-incurred immaturity.

We assume a common sense as the necessary condition of the universal communicability of our knowledge, which is presupposed in every logic and every principle of knowledge that is not one of skepticism.

I feel a complete thirst for knowledge and an eager unrest to go further in it as well as satisfaction at every acquisition. There was a time when I believed that this alone could constitute the honor of mankind, and I had contempt for the ignorant rabble who know nothing.

Criticism alone can sever the root of materialism, fatalism, atheism, free-thinking, fanaticism, and superstition, which can be injurious universally; as well as of idealism and skepticism, which are dangerous chiefly to the Schools, and hardly allow of being handed on to the public.

I assert that, in any particular natural science, one encounters genuine scientific substance only to the extent that mathematics is present.