“Act only according to that maxim whereby you can at the same time will that it should become a universal law.”
— Immanuel Kant, Groundwork of the Metaphysics of Morals
What this quote means
The first formulation of the categorical imperative, stated in Kant's own words. It turns morality into a test of universalisability: could the principle behind your action hold for everyone, everywhere, without contradiction?
The idea behind it
The Categorical Imperative — Kant's supreme moral rule: act only on a principle you could will everyone to follow, and treat people as ends, never merely as means.
Groundwork is public domain; attribution verified. Confidence: high.