In Section II. the passage is made from the popular moral philosophy thus arising to the metaphysic of morals.  He denies that the notion of duty that has been taken above from common sage is empirical.  It is proved not to be such from the very assertions of philosophers that men always act from more or less refined self-love; assertions that are founded upon the difficulty of proving that acts most apparently conformed to duty are really such.  The fact is, no act can be proved by experience to be absolutely moral, i.e., done solely from regard to duty, to the exclusion of all inclination; and therefore to concede that morality and duty are ideas to be had from experience, is the surest way to get rid of them altogether.  Duty, and respect for its law, are not to be preserved at all, unless Reason is allowed to lay absolute injunctions on the will, whatever experience says of their non-execution.  How, indeed, is experience to disclose a moral law, that, in applying to all rational beings as well as men, and to men only as rational, must originate a priori in pure (practical) Reason?  Instead of yielding the principles of morality, empirical examples of moral conduct have rather to be judged by these.

All supreme principles of morality, that are genuine, must rest on pure Reason solely; and the mistake of the popular practical philosophies in vogue, one and all—­whether advancing as their principle a special determination of human nature, or Perfection, or Happiness, or Moral Feeling, or Fear of God, or a little of this and a little of that—­is that there has been no previous consideration whether the principles of morality are to be sought for in our empirical knowledge of human nature at all.  Such consideration would have shown them to be altogether a priori, and would have appeared as a pure practical philosophy or metaphysic of morals (upon the completion of which any popularizing might have waited), kept free from admixture of Anthropology, Theology, Physics, Hyperphysics, &c., and setting forth the conception of Duty as purely rational, without the confusion of empirical motives.  To a metaphysic of this kind, Kant is now to ascend from the popular philosophy, with its stock-in-trade of single instances, following out the practical faculty of Reason from the general rules determining it, to the point where the conception of Duty emerges.

While things in nature work according to laws, rational beings alone can act according to a conceived idea of laws, i.e., to principles.  This is to have a Will, or, what is the same, Practical Reason, reason being required in deducing actions from laws.  If the Will follows Reason exactly and without fail, actions objectively necessary are necessary also subjectively; if, through subjective conditions (inclinations, &c.), the Will does not follow Reason inevitably, objectively necessary actions become subjectively contingent, and