The Critique of Practical Reason
Kant's "Critique of Practical Reason" ("Kritik der praktischen Vernunft"), published in 1788, is one of the most striking disquisitions in the whole range of German metaphysical literature. One of its paragraphs has alone sufficed to render it famous. The passage concerning the starry heavens and the moral law as the two transcendently overwhelming phenomena of the universe is, perhaps, more frequently quoted than any other written by a German author. This is the treatise which forms the central focus of Kant's thinking. It stands midway between the "Critique of Pure Reason" and the "Critique of Judgment." Herein Kant takes up the position of a vindicator of the truth of Christianity, approaching his proof of its validity and authority by first establishing positive affirmations of the immortality of the soul and the existence of God. It also includes a theory of happiness, and an argument concerning the summum bonum of life, the special aim being to demonstrate that man should not simply seek to be happy, but should, by absolute obedience to the moral law, seek to become worthy of that happiness which God can bestow.
I.—Analytic of Practical Reason
Practical principles are propositions containing a general determination of the will. They are maxims, or subjective propositions, when expressing the will of an individual; objective, when they are valid expressions of the will of rational beings generally.
Practical principles which pre-suppose an object of desire are empirical, or experimental, and supply no practical laws. Reason, in the scope of a practical law, influences the will not by the medium of pleasure or pain. All rational beings necessarily wish for happiness, but they are not all agreed either as to the means to attain it, or as to the objects of their enjoyment of it. Thus, subjective practical principles can only be reckoned as maxims, never as law.
A rational being ought not to conceive that his individual maxims are calculated to constitute universal laws, and to become the basis of universal legislation. To discover any law which would bring all men into harmony is absolutely impossible.
One of the problems of practical reason is to find the law which can necessarily determine the will, assuming that the will is free. The solution of this problem is to be found in action according to the moral law. We should so act that the maxim of our will can always be valid as a principle of universal legislation. Experience shows how the moral consciousness determines freedom of the will.
Suppose that someone affirms of his inclination for sensual pleasure that he cannot possibly resist temptation to indulgence. If a gallows were erected at the place where he is tempted, on which he should be hanged immediately after satiating his passions, would he not be able to control his inclination? We need not long doubt what would be his answer.
But ask him, if his sovereign commanded him to bear false witness against an honourable man, under penalty of death, whether he would hold it possible to conquer his love of life. He might not venture to say what he would choose, but he would certainly admit that it is possible to make choice. Thus, he judges that he can choose to do a thing because he is conscious of moral obligation, and he thus recognises for himself a freedom of will of which, but for the moral law, he would never have been conscious.