To define this idea practically, i.e., sufficiently for the maxims of our rational conduct, is the business of practical wisdom, and this again as a science is philosophy, in the sense in which the word was understood by the ancients, with whom it meant instruction in the conception in which the summum bonum was to be placed, and the conduct by which it was to be obtained.It would be well to leave this word in its ancient signification as a doctrine of the summum bonum, so far as reason endeavours to make this into a science.For on the one band the restriction annexed would suit the Greek expression (which signifies the love of wisdom), and yet at the same time would be sufficient to embrace under the name of philosophy the love of science: that is to say, of all speculative rational knowledge, so far as it is serviceable to reason, both for that conception and also for the practical principle determining our conduct, without letting out of sight the main end, on account of which alone it can be called a doctrine of practical wisdom.On the other hand, it would be no harm to deter the self-conceit of one who ventures to claim the title of philosopher by holding before him in the very definition a standard of self-estimation which would very much lower his pretensions.For a teacher of wisdom would mean something more than a scholar who has not come so far as to guide himself, much less to guide others, with certain expectation of attaining so high an end: it would mean a master in the knowledge of wisdom, which implies more than a modest man would claim for himself.Thus philosophy as well as wisdom would always remain an ideal, which objectively is presented complete in reason alone, while subjectively for the person it is only the goal of his unceasing endeavours; and no one would be justified in professing to be in possession of it so as to assume the name of philosopher who could not also show its infallible effects in his own person as an example (in his self-mastery and the unquestioned interest that he takes pre-eminently in the general good), and this the ancients also required as a condition of deserving that honourable title.
We have another preliminary remark to make respecting the dialectic of the pure practical reason, on the point of the definition of the summum bonum (a successful solution of which dialectic would lead us to expect, as in case of that of the theoretical reason, the most beneficial effects, inasmuch as the self-contradictions of pure practical reason honestly stated, and not concealed, force us to undertake a complete critique of this faculty).
The moral law is the sole determining principle of a pure will.
But since this is merely formal (viz., as prescribing only the form of the maxim as universally legislative), it abstracts as a determining principle from all matter that is to say, from every object of volition.Hence, though the summum bonum may be the whole object of a pure practical reason, i.e., a pure will, yet it is not on that account to be regarded as its determining principle; and the moral law alone must be regarded as the principle on which that and its realization or promotion are aimed at.This remark is important in so delicate a case as the determination of moral principles, where the slightest misinterpretation perverts men's minds.For it will have been seen from the Analytic that, if we assume any object under the name of a good as a determining principle of the will prior to the moral law and then deduce from it the supreme practical principle, this would always introduce heteronomy and crush out the moral principle.
It is, however, evident that if the notion of the summum bonum includes that of the moral law as its supreme condition, then the summum bonum would not merely be an object, but the notion of it and the conception of its existence as possible by our own practical reason would likewise be the determining principle of the will, since in that case the will is in fact determined by the moral law which is already included in this conception, and by no other object, as the principle of autonomy requires.This order of the conceptions of determination of the will must not be lost sight of, as otherwise we should misunderstand ourselves and think we had fallen into a contradiction, while everything remains in perfect harmony.