To argue with a man who has renounced the use and authority of reason is like administering medicine to the dead.

Full quote:
To argue with a man who has renounced the use and authority of reason, and whose philosophy consists in holding humanity in contempt, is like administering medicine to the dead, or endeavoring to convert an atheist by scripture. Enjoy, sir, your insensibility of feeling and reflecting. It is the prerogative of animals. And no man will envy you these honors, in which a savage only can be your rival and a bear your master.