It is by his activities and not by enjoyment that man feels he is alive. In idleness we not only feel that life is fleeting, but we also feel lifeless.

Thoughts without content are empty, intuitions without concepts are blind.

A categorical imperative would be one which represented an action as objectively necessary in itself, without reference to any other purpose.

He who is cruel to animals becomes hard also in his dealings with men. We can judge the heart of a man by his treatment of animals.

[A ruler is merely] the trustee of the rights of other men and he must always stand in dread of having in some way violated these rights.

All our knowledge begins with the senses, proceeds then to the understanding, and ends with reason. There is nothing higher than reason.

Act so that the maxim of your act could be made the principle of a universal law.