Enlightenment is man's leaving his self-caused immaturity. Immaturity is the incapacity to use one's intelligence without the guidance of another. Such immaturity is self-caused if it is not caused by lack of intelligence, but by lack of determination and courage to use one's intelligence without being guided by another. Sapere Aude! Have the courage to use your own intelligence! is therefore the motto of the enlightenment.

Art is purposiveness without purpose.

Out of the crooked timber of humanity, no straight thing was ever made.

Act that your principle of action might safely be made a law for the whole world.

Metaphysics is a dark ocean without shores or lighthouse, strewn with many a philosophic wreck.

It is precisely in knowing its limits that philosophy consists.

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.