Internal: Perfection. External: Will of God.
The subjective elements are all experimental, or empirical, and cannot supply the universal principle of morality, though they are expounded in that sense by such writers as Montaigne, Mandeville, Epicurus, and Hutcheson.
But the objective elements, as enunciated and expounded by Wolf and the Stoics, and by Crusius and other theological moralists, are founded on reason, for absolute perfection as a quality of things (that is, God Himself) can only be thought of by rational concepts.
The conception of perfection in a practical sense is the adequacy of a thing for various ends. As a human quality (and so internal) this is simply talent, and what completes it is skill. But supreme perfection in substance, that is, God Himself, and therefore external (considered practically), is the adequacy of this being for all purposes. All these principles above classified are material, and so can never furnish the supreme moral law. For even the Divine will can supply a motive in the human mind because of the expectation of happiness from it.
Therefore, the formal practical principle of the pure reason insists that the mere form of a universal legislation must constitute the ultimate determining principle of the will. Here is the only possible practical principle which is sufficient to furnish categorical imperatives, that is, practical laws which make action a duty.
It follows from this analytic that pure reason can be practical. It can determine the will independently of all merely experimental elements.
There is a remarkable contrast between the working of the pure speculative reason and that of the pure practical reason. In the former—as was shown in the treatise on that subject—a pure, sensible intuition of time and space made knowledge possible, though only of objects of the senses.
On the contrary, the moral law brings before us a fact absolutely inexplicable from any of the data of the world of sense. And the entire range of our theoretical use of reason indicates a pure world of understanding, which even positively determines it, and enables us to know something of it—namely, a law.
We must observe the distinction between the laws of a system of nature to which the will is subject, and of a system of nature which is subject to the will. In the former, the objects cause the ideas which determine the will; in the latter, the objects are caused by the will. Hence, causality of the will has its determining principle exclusively in the faculty of pure reason, which may, therefore, also be called a pure practical reason.
The moral law is a law of the causality through freedom, and therefore of the possibility of a super-sensible system of nature. It determines the will by imposing on its maxim the condition of a universal legislative form, and thus it is able for the first time to impart practical reality to reason, which otherwise would continue to be transcendent when seeking to proceed speculatively with its ideas.