Immanuel Kant

Act only according to that maxim whereby you can, at the same time, will that it should become a universal law.

Dare to know! Have the courage to use your own reason.

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

We see things not as they are, but as we are.

Happiness is not an ideal of reason, but of imagination.

The busier we are, the more acutely we feel that we live, the more conscious we are of life.