That the absolutely good Will must be autonomous—­i.e., without any kind of motive or interest, lay Commands on itself that are at the same time fit to be laws for all rational beings, appears, then, from a deeper consideration of even the popular conceptions of morality.  But now the question can no longer be put off:  Is Morality, of which this is the only conception, a reality or a phantom?  All the different expressions given to the Categorical Imperatives are synthetic practical propositions a priori; they postulate a possible synthetic use of the pure practical reason.  Is there, and how is there, such a possible synthetic use?  This is the question (the same as the other) that Kant proceeds to answer in the Third Section, by giving, in default of a complete Critique of the faculty, as much as is necessary for the purpose.  But here, since he afterwards undertook the full Critique, it is better to stop the analysis of the earlier work, and summarily draw upon both for the remainder of the argument, and the rather because some important points have to be added that occur only in the later treatise.  The foregoing is a sufficient example of his method of treatment.

The synthetic use of the pure practical reason, in the Categorical Imperative, is legitimized; Autonomy of the Will is explained; Duty is shown to be no phantom—­through the conception of Freedom of Will, properly understood.  Theoretically (speculatively), Freedom is undemonstrable; being eternally met, in one of the (cosmological) Antinomies of the Pure Reason, by the counter-assertion that everything in the universe takes place according to unchanging laws of nature.  Even theoretically, however, Freedom is not inconceivable, and morally we become certain of it; for we are conscious of the ‘ought’ of duty, and with the ‘ought’ there must go a ‘can.’  It is not, however, as Phenomenon or Sensible Ens that a man ‘can,’ is free, has an absolute initiative; all phenomena or Sensible Entia, being in space and time, are subject to the Natural Law of Causality.  But man is also Noumenon, Thing-in-self, Intelligible Ens; and as such, being free from conditions of time and space, stands outside of the sequence of Nature.  Now, the Noumenon or Ens of the Reason (he assumes) stands higher than, or has a value above, the Phenomenon or Sensible Ens (as much as Reason stands higher than Sense and Inclination); accordingly, while it is only man as Noumenon that ‘can,’ it is to man as Phenomenon that the ‘ought’ is properly addressed; it is upon man as Phenomenon that the law of Duty, prescribed, with perfect freedom from motive, by Man as Noumenon, is laid.