Rational beings have the unique capacity to cause events through free will. Since a will that is free must be a will that gives itself its own law, autonomy of the will and free will are one and the same. Thus a will is free when and only when it follows moral laws. Morality therefore follows from the concept of free will.
Since morality is a matter for all rational beings, not just human beings, we cannot base our notion of morality on the concept of free will unless we establish that all rational beings have free will. "Proving" this issue from experience would be difficult if not impossible, yet we may assume that a being really is free if it thinks of itself as free when it acts, for such a being must be aware of morality's demands whether or not it really is free to execute them. Moreover, any being endowed with reason and with a will must think of itself as free, for reason would not be reason if it were subject to control by irrational forces from outside itself.
Thus we may presuppose that rational beings think of themselves as free, and we have established that the moral law and the categorical imperative follow from this concept of freedom. Yet why we would want to abide by this law is a different question. We may want to be moral because we feel it makes us more deserving of future happiness, yet this is merely an expression for the value that we attribute to morality; it does not answer the question of why we value morality.
Our logic so far seems circular: we explained that we may think of ourselves as free because we are aware of moral demands, yet on the other hand we based our notion of morality on our concept of freedom. This problem may be resolved by drawing a distinction between "appearances" and "things in themselves." In our everyday experience, we encounter a "sensible world" of appearances. We may presume that these appearances come from real objects ("things in themselves"), but we can have knowledge of these objects only insofar as they affect us. Human beings make use of the faculty of "understanding" to make sense of the world of appearances. The faculty of reason distinguishes between the "sensible" world of appearances and experiences, which will be different for all individuals, and the "intelligible" world of concepts that make sense to all people. Reason may also recognize the limits of understanding.
Human beings may understand themselves from the perspective of either the sensible or the intelligible world: they may think of themselves in terms of either the laws of nature, or the laws of reason. Insofar as rational beings think of themselves in terms of the laws of reason, they understand themselves to possess a free will that is independent of the forces of nature that govern the sensible world. This idea of freedom is the basis for the concept of autonomy and the moral law. Thus our inferences are not circular: our concept of freedom does not depend on our notion of morality; rather, it may be derived from our participation in the intelligible world.
If people lived exclusively in the intelligible world, they would possess a perfectly free and autonomous will. On the other hand, if people lived exclusively in the sensible world, all their actions would be governed by the law of nature and the rules of cause and effect. When actions of the will enter the sensible world, they have to be understood in terms of the rules of cause and effect that govern that world; thus actions will appear to have been caused by material needs and inclinations. Nevertheless, as rational beings we know that the intelligible world is the primary world for us; it is the "ground" for the sensible world, for we know of our sensible self only through appearances, whereas we have immediate knowledge of our intelligible self. We therefore know that we are subject to the categorical imperative and the ideas of freedom and morality that are entailed by the intelligible world.