Thus good and evil properly relate to actions, not to personal sensations. So, if anything is to be reckoned simply good or evil, it can only be so estimated by the way of acting. Hence, only the maxim of the will, and consequently the person himself, can be called good or evil, not the thing itself.
The Stoic was right, even though he might be laughed at, who during violent attacks of gout exclaimed, "Pain, I will never admit that thou art an evil!" What he felt was indeed what we call a bad thing; but he had no reason to admit that any evil attached thereby to himself, for the pain did not in the least detract from his personal worth, but only from that of his condition. If a single lie had been on his conscience it would have humiliated his soul; but pain seemed only to elevate it, when he was not conscious of having deserved it as a punishment for any unjust deed.
The rule of judgment subject to the laws of pure practical reason is this: Ask yourself whether if the action you propose were to happen by a natural system of law, of which you were yourself a part, you could regard it as possible by your own will? In fact, everyone does decide by this rule whether actions are morally good or evil.
II.—Dialectic of Practical Reason
THE IMMORTALITY OF THE SOUL
Pure practical reason postulates the immortality of the soul, for reason in the pure and practical sense aims at the perfect good (summum bonum), and this perfect good is only possible on the supposition of the soul's immortality. It is the moral law which determines the will, and, in this will, the perfect harmony of the mind with the moral law, is the supreme condition of the summum bonum. The principle of the moral destination of our nature—that only by endless progress can we come into full harmony with the moral law—is of the greatest use, not only for fortifying the speculative reason, but also with respect to religion. In default of this, either the moral law is degraded from its holiness, being represented as indulging our convenience, or else men strain after an unattainable aim, hoping to gain absolute holiness of will, thus losing themselves in fanatical theosophic dreams utterly contradicting self-knowledge.
For a rational, but finite, being the only possibility is an endless progression from the lower to the higher degrees of perfection. The Infinite Being, to whom the time-condition is nothing, sees in this endless succession the perfect harmony with the moral law.
THE EXISTENCE OF GOD
The pure practical reason must also postulate the existence of God as the necessary condition of the attainment of the summum bonum. As the perfect good can only be promoted by accordance of the will with the moral law, so also this summum bonum is possible only through the supremacy of an Infinite Being possessed of causality harmonising with morality. But the postulate of the highest derived good (sometimes denominated the best world) coincides with the postulate of a highest original good, or of the existence of God.
We now perceive why the Greeks could never solve their problem of the possibility of the summum bonum, because they made the freedom of the human will the only and all-sufficient ground of happiness, imagining there was no need for the existence of God for that end. Christianity alone affords an idea of the summum bonum which answers fully to the requirement of practical reason. That idea is the Kingdom of God.