As a rational being, and consequently belonging to the intelligible world, man can never conceive the causality of his own will otherwise than on condition of the idea of freedom. for independence on the determining causes of the sensible world (an independence which Reason must always ascribe to itself) is freedom.  Now the idea of freedom is inseparably connected with the conception of autonomy, and this again with the universal principle of morality which is ideally the foundation of all actions of rational beings, just as the law of nature is of all phenomena.

Now the suspicion is removed which we raised above, that there was a latent circle involved in our reasoning from freedom to autonomy, and from this to the moral law, viz.:  that we laid down the idea of freedom because of the moral law only that we might afterwards in turn infer the latter from freedom and that consequently we could assign no reason at all for this law, but could only [present] [Footnote:  The verb is wanting in the original.] it as a petitio principii which well disposed minds would gladly concede to us, but which we could never put forward as a provable proposition.  For now we see that when we conceive ourselves as free we transfer ourselves into the—­world of understanding as members of it, and recognise the autonomy of the will with its consequence, morality; whereas, if we conceive ourselves as under obligation we consider ourselves as belonging to the world of sense, and at the same time to the world of understanding.

How is a Categorical Imperative Possible?

Every rational being reckons himself qua intelligence as belonging to the world of understanding, and it is simply as an efficient cause belonging to that world that he calls his causality a will.  On the other side he is also conscious of himself as a part of the world of sense in which his actions which are mere appearances [phenomena] of that causality are displayed; we cannot, however, discern how they are possible from this causality which we do not know; but instead of that, these actions as belonging to the sensible world must be viewed as determined by other phenomena, namely,—­desires and inclinations.  If therefore I were only a member of the world of understanding, then all my actions would perfectly conform to the principle of autonomy of the pure will; if I were only a part of the world of sense they would necessarily be assumed to conform wholly to the natural law of desires and inclinations, in other words, to the heteronomy of nature. (The former would rest on morality as the supreme principle, the latter on happiness.), Since, however, the world of understanding contains the foundation of the world of sense, and consequently of its laws alsof and accordingly gives the law to my will (which belongs wholly to the world of understanding) directly, and must be conceived as doing so, it follows that, although on the one side I must regard myself as a being belonging to the world of sense, yet on the other side I must recognise myself as subject as an intelligence to the law of the world of understanding, i. e., to reason, which contains this law in the idea of freedom, and therefore as subject to the autonomy of the will:  consequently I must regard the laws of the world of understanding as imperatives for me, and the actions which conform to them as duties.