The Critique of Practical Reason [49]
the unity of the whole faculty of reason (theoretical as well as practical) and be able to derive all from one principle, which, is what human reason inevitably demands, as it finds complete satisfaction only in a perfectly systematic unity of its knowledge. If now we consider also the contents of the knowledge that we can have of a pure practical reason, and by means of it, as shown by the Analytic, we find, along with a remarkable analogy between it and the theoretical, no less remarkable differences. As regards the theoretical, the faculty of a pure rational cognition a priori could be easily and evidently proved by examples from sciences (in which, as they put their principles to the test in so many ways by methodical use, there is not so much reason as in common knowledge to fear a secret mixture of empirical principles of cognition). But, that pure reason without the admixture of any empirical principle is practical of itself, this could only be shown from the commonest practical use of reason, by verifying the fact, that every man's natural reason acknowledges the supreme practical principle as the supreme law of his will- a law completely a priori and not depending on any sensible data. It was necessary first to establish and verify the purity of its origin, even in the judgement of this common reason, before science could take it in hand to make use of it, as a fact, that is, prior to all disputation about its possibility, and all the consequences that may be drawn from it. But this circumstance may be readily explained from what has just been said; because practical pure reason must necessarily begin with principles, which therefore must be the first data, the foundation of all science, and cannot be derived from it. It was possible to effect this verification of moral principles as principles of a pure reason quite well, and with sufficient certainty, by a single appeal to the judgement of common sense, for this reason, that anything empirical which might slip into our maxims as a determining principle of the will can be detected at once by the feeling of pleasure or pain which necessarily attaches to it as exciting desire; whereas pure practical reason positively refuses to admit this feeling into its principle as a condition. The heterogeneity of the determining principles (the empirical and rational) is clearly detected by this resistance of a practically legislating reason against every admixture of inclination, and by a peculiar kind of sentiment, which, however, does not precede the legislation of the practical reason, but, on the contrary, is produced by this as a constraint, namely, by the feeling of a respect such as no man has for inclinations of whatever kind but for the law only; and it is detected in so marked and prominent a manner that even the most uninstructed cannot fail to see at once in an example presented to him, that empirical principles of volition may indeed urge him to follow their attractions, but that he can never be expected to obey anything but the pure practical law of reason alone. The distinction between the doctrine of happiness and the doctrine of morality, in the former of which empirical principles constitute the entire foundation, while in the second they do not form the smallest part of it, is the first and most important office of the Analytic of pure practical reason; and it must proceed in it with as much exactness and, so to speak, scrupulousness, as any geometer in his work. The philosopher, however, has greater difficulties to contend with here (as always in rational cognition by means of concepts merely without construction), because he cannot take any intuition as a foundation (for a pure noumenon). He has, however, this advantage that, like the chemist, he can at any time make an experiment with every man's practical reason for the purpose of distinguishing the moral (pure) principle of determination from the empirical; namely, by adding the moral law (as a determining principle) to the empirically affected will (e.g., that of the man who would be ready to lie