They all proceed from the principle of morality, which is not a postulate but a law, by which reason determines the will directly, which will, because it is so determined as a pure will, requires these necessary conditions of obedience to its precept. These postulates are not theoretical dogmas but, suppositions practically necessary; while then they do [not] extend our speculative knowledge, they give objective reality to the ideas of speculative reason in general (by means of their reference to what is practical), and give it a right to concepts, the possibility even of which it could not otherwise venture to affirm.
These postulates are those of immortality, freedom positively considered (as the causality of a being so far as he belongs to the intelligible world), and the existence of God. The first results from the practically necessary condition of a duration adequate to the complete fulfilment of the moral law; the second from the necessary supposition of independence of the sensible world, and of the faculty of determining one's will according to the law of an intelligible world, that is, of freedom; the third from the necessary condition of the existence of the summum bonum in such an intelligible world, by the supposition of the supreme independent good, that is, the existence of God.
Thus the fact that respect for the moral law necessarily makes the summum bonum an object of our endeavours, and the supposition thence resulting of its objective reality, lead through the postulates of practical reason to conceptions which speculative reason might indeed present as problems, but could never solve. Thus it leads: 1. To that one in the solution of which the latter could do nothing but commit paralogisms (namely, that of immortality), because it could not lay hold of the character of permanence, by which to complete the psychological conception of an ultimate subject necessarily ascribed to the soul in self-consciousness, so as to make it the real conception of a substance, a character which practical reason furnishes by the postulate of a duration required for accordance with the moral law in the summum bonum, which is the whole end of practical reason. 2. It leads to that of which speculative reason contained nothing but antinomy, the solution of which it could only found on a notion problematically conceivable indeed, but whose objective reality it could not prove or determine, namely, the cosmological idea of an intelligible world and the consciousness of our existence in it, by means of the postulate of freedom (the reality of which it lays down by virtue of the moral law), and with it likewise the law of an intelligible world, to which speculative reason could only point, but could not define its conception. 3. What speculative reason was able to think, but was obliged to leave undetermined as a mere transcendental ideal, viz., the theological conception of the first Being, to this it gives significance (in a practical view, that is, as a condition of the possibility of the object of a will determined by that law), namely, as the supreme principle of the summum bonum in an intelligible world, by means of moral legislation in it invested with sovereign power.
Is our knowledge, however, actually extended in this way by pure practical reason, and is that immanent in practical reason which for the speculative was only transcendent? Certainly, but only in a practical point of view. For we do not thereby take knowledge of the nature of our souls, nor of the intelligible world, nor of the Supreme Being, with respect to what they are in themselves, but we have merely combined the conceptions of them in the practical concept of the summum bonum as the object of our will, and this altogether a priori, but only by means of the moral law, and merely in reference to it, in respect of the object which it commands. But how freedom is possible, and how we are to conceive this kind of causality theoretically and positively, is not thereby discovered; but only that there is such a causality is postulated by the moral law and in its behoof. It is the same with the remaining ideas, the possibility of which no human intelligence will ever fathom, but the truth of which, on the other hand, no sophistry will ever wrest from the conviction even of the commonest man.
BOOK2|CHAPTER2 ^paragraph 65
VII. How is it possible to conceive an Extension of Pure Reason in a Practical point of view, without its Knowledge as Speculative being enlarged at the same time?
BOOK2|CHAPTER2 ^paragraph 70
In order not to be too abstract, we will answer this question at once in its application to the present case. In order to extend a pure cognition practically, there must be an a priori purpose given, that is, an end as object (of the will), which independently of all theological principle is presented as practically necessary by an imperative which determines the will directly (a categorical imperative), and in this case that is the summum bonum. This, however, is not possible without presupposing three theoretical conceptions (for which, because they are mere conceptions of pure reason, no corresponding intuition can be found, nor consequently by the path of theory any objective reality); namely, freedom, immortality, and God. Thus by the practical law which commands the existence of the highest good possible in a world, the possibility of those objects of pure speculative reason is postulated, and the objective reality which the latter could not assure them. By this the theoretical knowledge of pure reason does indeed obtain an accession; but it consists only in this, that those concepts which otherwise it had to look upon as problematical (merely thinkable) concepts, are now shown assertorially to be such as actually have objects; because practical reason indispensably requires their existence for the possibility of its object, the summum bonum, which practically is absolutely necessary, and this justifies theoretical reason in assuming them. But this extension of theoretical reason is no extension of speculative, that is, we cannot make any positive use of it in a theoretical point of view. For as nothing is accomplished in this by practical reason, further than that these concepts are real and actually have their (possible) objects, and nothing in the way of intuition of them is given thereby (which indeed could not be demanded), hence the admission of this reality does not render any synthetical proposition possible. Consequently, this discovery does not in the least help us to extend this knowledge of ours in a speculative point of view, although it does in respect of the practical employment of pure reason. The above three ideas of speculative reason are still in themselves not cognitions; they are however (transcendent) thoughts, in which there is nothing impossible. Now, by help of an apodeictic practical law, being necessary conditions of that which it commands to be made an object, they acquire objective reality; that is, we learn from it that they have objects, without being able to point out how the conception of them is related to an object, and this, too, is still not a cognition of these objects; for we cannot thereby form any synthetical judgement about them, nor determine their application theoretically; consequently, we can make no theoretical rational use of them at all, in which use all speculative knowledge of reason consists. Nevertheless, the theoretical knowledge, not indeed of these objects, but of reason generally, is so far enlarged by this, that by the practical postulates objects were given to those ideas, a merely problematical thought having by this means first acquired objective reality. There is therefore no extension of the knowledge of given supersensible objects, but an extension of theoretical reason and of its knowledge in respect of the supersensible generally; inasmuch as it is compelled to admit that there are such objects, although it is not able to define them more closely, so as itself to extend this knowledge of the objects (which have now been given it on practical grounds, and only for practical use). For this accession, then, pure theoretical reason, for which all those ideas are transcendent and without object, has simply to thank its practical faculty. In this they become immanent and constitutive, being the source of the possibility of realizing the necessary object of pure practical reason (the summum bonum); whereas apart from this they are transcendent, and merely regulative principles of speculative reason, which do not require it to assume a new object beyond experience, but only to bring its use in experience nearer to completeness. But when once reason is in possession of this accession, it will go to work with these ideas as speculative reason (properly only to assure the certainty of its practical use) in a negative manner: that is, not extending but clearing up its knowledge so as on one side to keep off anthropomorphism, as the source of superstition, or seeming extension of these conceptions by supposed experience; and on the other side fanaticism, which promises the same by means of supersensible intuition or feelings of the like kind. All these are hindrances to the practical use of pure reason, so that the removal of them may certainly be considered an extension of our knowledge in a practical point of view, without contradicting the admission that for speculative purposes reason has not in the least gained by this.