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.]

Moral perfection thus provided for, God must be postulated in order to find the ground of the required conjunction of Felicity.  Happiness is the condition of the rational being in whose whole existence everything goes according to wish and will; and this is not the condition of man, for in him observance of the moral law is not conjoined with power of disposal over the laws of nature.  But, as Practical Reason demands the conjunction, it is to be found only in a being who is the author at once of Nature and of the Moral Law; and this is God. [The same remark once more applies, that here what is obtained is a moral certainty of the existence of the Deity:  the negative result of the Critique of the Pure (speculative) Reason abides what it was.]

We may now attempt to summarize this abstruse Ethical theory of Kant.

I—–­The STANDARD of morally good action (or rather Will), as expressed in the different forms of the Categorical Imperative, is the possibility of its being universally extended as a law for all rational beings.  His meaning comes out still better in the obverse statement:  The action is bad that cannot be, or at least cannot be wished to lie, turned unto a universal law.

II.—­Kant would expressly demur to being questioned as to his PSYCHOLOGY of Ethics; since he puts his own theory in express opposition to every other founded upon any empirical view of the mental constitution.  Nevertheless, we may extract some kind of answers to the usual queries.