Before acting, ask whether you'd be fine if literally everyone acted that way

Immanuel Kant argued an action is only morally permissible if you could will the rule behind it to become a universal law followed by everyone, without contradiction. Lying to get out of a promise fails the test, Kant argued, because if everyone lied whenever convenient, promises themselves would stop meaning anything, undermining the very lie you were trying to tell.

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