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. _Freedom of Will_ in Man as Rational End or Thing-in-self is thus the great Postulate of the pure Practical Reason; we can be sure of the fact (although it must always remain speculatively undemonstrable), because else there could be no explanation of the Categorical Imperative of Duty. But inasmuch as the Practical Reason, besides enjoining a law of Duty, must provide also a final end of action in the idea of an unconditioned Supreme Good, it contains also two other Postulates: Man being a sentient as well as a rational being, Happiness as well as Perfect Virtue or Moral Perfection must enter into the Summum Bonum (not, one of them to the exclusion of the other, as the Stoics and Epicureans, in different senses, declared). Now, since there is no such necessary conjunction of the two in nature, it must be sought otherwise. It is found in postulating _Immortality_ and _God_. _Immortality_ is required to render possible the attainment of moral perfection. Virtue out of _respect_ for law, with a constant tendency to fall away, is all that is attainable in life. The _Holiness_, or complete accommodation of the will to the Moral Law, implied in the Summum Bonum, can be attained to only in the course of an infinite progression; which means personal Immortality. [As in the former case, the _speculative_ impossibility of proving the immateriality, &c., of the supernatural soul is not here overcome; but Immortality is _morally_ certain, being demanded by the Practical Reason.]