Die Metaphysik der Sitten

If there exists on any subject a philosophy (that is, a system of rational knowledge based on concepts), then there must also be for this philosophy a system of pure rational concepts, independent of any condition of intuition, in other words, a metaphysic. It may be asked whether metaphysical elements are required also for every practical philosophy, which is the doctrine of duties, and therefore also for Ethics, in order to be able to present it as a true science (systematically), not merely as an aggregate of separate doctrines (fragmentarily). As regards pure jurisprudence, no one will question this requirement; for it concerns only what is formal in the elective will, which has to be limited in its external relations according to laws of freedom; without regarding any end which is the matter of this will. Here, therefore, deontology is a mere scientific doctrine (doctrina scientiae). * * One who is acquainted with practical philosophy is not, therefore, a practical philosopher. The latter is he who makes the rational end the principle of his actions, while at the same time he joins with this the necessary knowledge which, as it aims at action, must not be spun out into the most subtile threads of metaphysic, unless a legal duty is in question; in which case meum and tuum must be accurately determined in the balance of justice, on the principle of equality of action and action, which requires something like mathematical proportion, but not in the case of a mere ethical duty.…