what exists in the nature of things is the standard of our judgement; what each man feels within himself is the standard of sentiment.
 David Hume, An Enquiry Concerning the Principles of Morals (1751). copy citation

Context

“the examples cited, the authorities appealed to, the analogies employed, the fallacies detected, the inferences drawn, and the several conclusions adjusted to their proper principles. Truth is disputable; not taste: what exists in the nature of things is the standard of our judgement; what each man feels within himself is the standard of sentiment. Propositions in geometry may be proved, systems in physics may be controverted; but the harmony of verse, the tenderness of passion, the brilliancy of wit, must give immediate pleasure. No man reasons concerning another's beauty;” source