This law (as far as rational beings are concerned) gives to the world of sense, which is a sensible system of nature, the form of a world of the understanding, that is, of a supersensible system of nature, without interfering with its mechanism.Now, a system of nature, in the most general sense, is the existence of things under laws.The sensible nature of rational beings in general is their existence under laws empirically conditioned, which, from the point of view of reason, is heteronomy.The supersensible nature of the same beings, on the other hand, is their existence according to laws which are independent of every empirical condition and, therefore, belong to the autonomy of pure reason.And, since the laws by which the existence of things depends on cognition are practical, supersensible nature, so far as we can form any notion of it, is nothing else than a system of nature under the autonomy of pure practical reason.Now, the law of this autonomy is the moral law, which, therefore, is the fundamental law of a supersensible nature, and of a pure world of understanding, whose counterpart must exist in the world of sense, but without interfering with its laws.We might call the former the archetypal world (natura archetypa), which we only know in the reason; and the latter the ectypal world (natura ectypa), because it contains the possible effect of the idea of the former which is the determining principle of the will.For the moral law, in fact, transfers us ideally into a system in which pure reason, if it were accompanied with adequate physical power, would produce the summum bonum, and it determines our will to give the sensible world the form of a system of rational beings.
The least attention to oneself proves that this idea really serves as the model for the determinations of our will.
When the maxim which I am disposed to follow in giving testimony is tested by the practical reason, I always consider what it would be if it were to hold as a universal law of nature.It is manifest that in this view it would oblige everyone to speak the truth.For it cannot hold as a universal law of nature that statements should be allowed to have the force of proof and yet to be purposely untrue.Similarly, the maxim which I adopt with respect to disposing freely of my life is at once determined, when I ask myself what it should be, in order that a system, of which it is the law, should maintain itself.It is obvious that in such a system no one could arbitrarily put an end to his own life, for such an arrangement would not be a permanent order of things.And so in all similar cases.
Now, in nature, as it actually is an object of experience, the free will is not of itself determined to maxims which could of themselves be the foundation of a natural system of universal laws, or which could even be adapted to a system so constituted; on the contrary, its maxims are private inclinations which constitute, indeed, a natural whole in conformity with pathological (physical) laws, but could not form part of a system of nature, which would only be possible through our will acting in accordance with pure practical laws.Yet we are, through reason, conscious of a law to which all our maxims are subject, as though a natural order must be originated from our will.
This law, therefore, must be the idea of a natural system not given in experience, and yet possible through *******; a system, therefore, which is supersensible, and to which we give objective reality, at least in a practical point of view, since we look on it as an object of our will as pure rational beings.
Hence the distinction between the laws of a natural system to which the will is subject, and of a natural system which is subject to a will (as far as its relation to its free actions is concerned), rests on this, that in the former the objects must be causes of the ideas which determine the will; whereas in the latter the will is the cause of the objects; so that its causality has its determining principle solely in the pure faculty of reason, which may therefore be called a pure practical reason.
There are therefore two very distinct problems: how, on the one side, pure reason can cognise objects a priori, and how on the other side it can be an immediate determining principle of the will, that is, of the causality of the rational being with respect to the reality of objects (through the mere thought of the universal validity of its own maxims as laws).
The former, which belongs to the critique of the pure speculative reason, requires a previous explanation, how intuitions without which no object can be given, and, therefore, none known synthetically, are possible a priori; and its solution turns out to be that these are all only sensible and, therefore, do not render possible any speculative knowledge which goes further than possible experience reaches; and that therefore all the principles of that pure speculative reason avail only to make experience possible; either experience of given objects or of those that may be given ad infinitum, but never are completely given.