The Critique of Practical Reason by Abbott, Thomas Kingsmill, 1829-1913, Kant, Immanuel, 1724-1804

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.

BOOK_2|CHAPTER_2