Kant, Immanuel. Critique of Practical Reason. Edited and translated by Mary J. Gregor. Cambridge: Cambridge University Press, 1997.

Excerpt:

“The theoretical use of reason was concerned with objects of the cognitive faculty only, and a critical examination of it with reference to this use applied properly only to the pure faculty of cognition; because this raised the suspicion, which was afterwards confirmed, that it might easily pass beyond its limits, and be lost among unattainable objects, or even contradictory notions. It is quite different with the practical use of reason. In this, reason is concerned with the grounds of determination of the will, which is a faculty either to produce objects corresponding to ideas, or to determine ourselves to the effecting of such objects (whether the physical power is sufficient or not); that is, to determine our causality. For here, reason can at least attain so far as to determine the will, and has always objective reality in so far as it is the volition only that is in question. The first question here then is, whether pure reason of itself alone suffices to determine the will, or whether it can be a ground of determination only as dependent on empirical conditions.

Now, here there comes in a notion of causality justified by the critique of the pure reason, although not capable of being presented empirically, viz. that of freedom; and if we can now discover means of proving that this property does in fact belong to the human will (and so to the will of all rational beings), then it will not only be shown that pure reason can be practical, but that it alone, and not reason empirically limited, is indubitably practical; consequently, we shall have to make a critical examination, not of pure practical reason, but only of practical reason generally. For when once pure reason is shown to exist it needs no critical examination. For reason itself contains the standard for the critical examination of every use of it. The critique, then, of practical reason generally is bound to prevent the empirically conditioned reason from claiming exclusively to furnish the ground of determination of the will. If it is proved that there is a [practical] reason, its employment is alone immanent; the empirically conditioned use, which claims supremacy, is on the contrary transcendent, and expresses itself in demands and precepts which go quite beyond its sphere. This is just the opposite of what might be said of pure reason in its speculative employment.”

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