The doctrine of Christianity,* even if we do not yet consider it as a religious doctrine, gives, touching this point, a conception of the summum bonum (the kingdom of God), which alone satisfies the strictest demand of practical reason.The moral law is holy (unyielding) and demands holiness of morals, although all the moral perfection to which man can attain is still only virtue, that is, a rightful disposition arising from respect for the law, implying consciousness of a constant propensity to transgression, or at least a want of purity, that is, a mixture of many spurious (not moral)motives of obedience to the law, consequently a self-esteem combined with humility.In respect, then, of the holiness which the Christian law requires, this leaves the creature nothing but a progress in infinitum, but for that very reason it justifies him in hoping for an endless duration of his existence.The worth of a character perfectly accordant with the moral law is infinite, since the only restriction on all possible happiness in the judgement of a wise and all powerful distributor of it is the absence of conformity of rational beings to their duty.But the moral law of itself does not promise any happiness, for according to our conceptions of an order of nature in general, this is not necessarily connected with obedience to the law.Now Christian morality supplies this defect (of the second indispensable element of the summum bonum) by representing the world in which rational beings devote themselves with all their soul to the moral law, as a kingdom of God, in which nature and morality are brought into a harmony foreign to each of itself, by a holy Author who makes the derived summum bonum possible.Holiness of life is prescribed to them as a rule even in this life, while the welfare proportioned to it, namely, bliss, is represented as attainable only in an eternity; because the former must always be the pattern of their conduct in every state, and progress towards it is already possible and necessary in this life; while the latter, under the name of happiness, cannot be attained at all in this world (so far as our own power is concerned), and therefore is made simply an object of hope.Nevertheless, the Christian principle of morality itself is not theological (so as to be heteronomy), but is autonomy of pure practical reason, since it does not make the knowledge of God and His will the foundation of these laws, but only of the attainment of the summum bonum, on condition of following these laws, and it does not even place the proper spring of this obedience in the desired results, but solely in the conception of duty, as that of which the faithful observance alone constitutes the worthiness to obtain those happy consequences.
*It is commonly held that the Christian precept of morality has no advantage in respect of purity over the moral conceptions of the Stoics; the distinction between them is, however, very obvious.The Stoic system made the consciousness of strength of mind the pivot on which all moral dispositions should turn; and although its disciples spoke of duties and even defined them very well, yet they placed the spring and proper determining principle of the will in an elevation of the mind above the lower springs of the senses, which owe their power only to weakness of mind.With them therefore, virtue was a sort of heroism in the wise man raising himself above the animal nature of man, is sufficient for Himself, and, while he prescribes duties to others, is himself raised above them, and is not subject to any temptation to transgress the moral law.All this, however, they could not have done if they had conceived this law in all its purity and strictness, as the precept of the Gospel does.When I give the name idea to a perfection to which nothing adequate can be given in experience, it does not follow that the moral ideas are thing transcendent, that is something of which we could not even determine the concept adequately, or of which it is uncertain whether there is any object corresponding to it at all, as is the case with the ideas of speculative reason; on the contrary, being types of practical perfection, they serve as the indispensable rule of conduct and likewise as the standard of comparison.Now if I consider Christian morals on their philosophical side, then compared with the ideas of the Greek schools, they would appear as follows: the ideas of the Cynics, the Epicureans, the Stoics, and the Christians are: simplicity of nature, prudence, wisdom, and holiness.In respect of the way of attaining them, the Greek schools were distinguished from one another thus that the Cynics only required common sense, the others the path of science, but both found the mere use of natural powers sufficient for the purpose.Christian morality, because its precept is framed (as a moral precept must be) so pure and unyielding, takes from man all confidence that be can be fully adequate to it, at least in this life, but again sets it up by enabling us to hope that if we act as well as it is in our power to do, then what is not in our power will come in to our aid from another source, whether we know how this may be or not.Aristotle and Plato differed only as to the origin of our moral conceptions.