Kant: only act on a rule you'd want everyone else to follow too

Immanuel Kant's categorical imperative says an action is only moral if you could will its underlying rule to become a universal law for everyone. Lying fails the test: if everyone lied whenever convenient, the trust that makes lying useful would collapse. Unlike consequentialism, it judges the rule, not the result.

— Immanuel Kant, Groundwork of the Metaphysics of Morals — 1785
Go deeper: get the book →