Act only as you'd will for everyone

In 1785, Kant proposed a single test for moral action: act only according to a rule you could will to become a universal law for everyone. Lying, he argued, fails instantly — a world where everyone lied would make lying itself impossible, since no one would trust anyone's word enough to be deceived by it.

— Immanuel Kant, Groundwork of the Metaphysics of Morals — 1785
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