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.

Thomas Paine (1737-1809) American political philosopher and writer

The American Crisis #5, “To General Sir William Howe” (23 Mar 1778)
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Sometimes shortened as: "To argue with a man who has renounced his reason is like giving medicine to the dead."