Kant said to act only on rules you'd want everyone else to follow too

Immanuel Kant argued that an action is only moral if you could will the principle behind it to become a universal law for everyone, without contradiction. Lying, for instance, fails the test: if everyone lied whenever convenient, the very idea of a trustworthy statement would collapse.

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