cannot itself prove their reality. The importance of these ideas is practical, i.e., connected with
morals. The purely intellectual use of reason leads to fallacies; its only right use is directed to
moral ends.
The practical use of reason is developed briefly near the end of The Critique of Pure Reason, and
more fully in The Critique of Practical Reason ( 1786). The argument is that the moral law
demands justice, i.e., happiness proportional to virtue. Only Providence can insure this, and has
evidently not insured it in this life. Therefore there is a God and a future life; and there must be
freedom, since otherwise there would be no such thing as virtue.
Kant ethical system, as set forth in his Metaphysic of Morals ( 1785), has considerable historical
importance. This book contains the "categorical imperative," which, at least as a phrase, is
familiar outside the circle of professional philosophers. As might be expected, Kant will have
nothing to do with utilitarianism, or with any doctrine which gives to morality a purpose outside
itself. He wants, he says, "a completely isolated metaphysic of morals, which is not mixed with
any theology or physics or hyperphysics." All moral concepts, he continues, have their seat and
origin wholly a priori in the reason. Moral worth exists only when a man acts from a sense of
duty; it is not enough that the act should be such as duty might have prescribed. The tradesman
who is honest from self-interest, or the man who is kind from benevolent impulse, is not virtuous.
The essence of morality is to be derived from the concept of law; for, though everything in nature
acts according to laws, only a rational being has the power of acting according to the idea of a law,
i.e., by Will. The idea of an objective principle, in so far as it is compelling to the will, is called a
command of the reason, and the formula of the command is called an imperative.
There are two sorts of imperative: the hypothetical imperative, which says "You must do so-and-
so if you wish to achieve such-andsuch an end"; and the categorical imperative, which says that a
certain kind of action is objectively necessary, without regard to any end. The categorical
imperative is synthetic and a priori. Its character is deduced by Kant from the concept of Law:
"If I think of a categorical imperative, I know at once what it contains. For as the imperative
contains, besides the Law, only the necessity of the maxim to be in accordance with this law, but
the Law