When I could have used a wife, I could not support one; and when I could support one, I no longer needed any

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

In every department of physical science there is only so much science, properly so-called, as there is mathematics.

Ours is an age of criticism, to which everything must be subjected. The sacredness of religion, and the authority of legislation, are by many regarded as grounds for exemption from the examination by this tribunal, But, if they are exempted, and cannot lay claim to sincere respect, which reason accords only to that which has stood the test of a free and public examination.

How then is perfection to be sought? Wherein lies our hope? In education, and in nothing else.

The more we come in contact with animals and observe their behaviour, the more we love them, for we see how great is their care of the young.

Have the courage to use your own reason- That is the motto of enlightenment. "Foundations of the Metaphysics of Morals" (1785)