Immanuel Kant holds in Foundations of the Metaphysics of Morals that ethics, like physics, is partly empirical and partly a priori. This work deals only with the a priori part in that it is based entirely on the use of reason without recourse to experience. Everyone must recognize, Kant writes, that since moral laws imply absolute necessity, they cannot be merely empirical. For example, “Thou shalt not lie” applies not merely to all human beings but to all rational beings. Its ground, therefore, must be found in pure reason. Moreover, what is done morally must be not only in accordance with law but also for the sake of law; if this were not its motivation, different circumstances of the agent would call forth different responses.
This book, issued as a preliminary to an intended metaphysic of morals, Kritik der praktischen Vernunft (1788; The Critique of Practical Reason, 1873), comprises a critical examination of purely practical reason and establishes the supreme principle of morality. The order of inquiry is from common moral knowledge to the supreme principle (analysis), then back to application in practice (synthesis).