Front Page Titles (by Subject) II.: of the propensity to evil in human nature. - Kant's Critique of Practical Reason and Other Works on the Theory of Ethics
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II.: of the propensity to evil in human nature. - Immanuel Kant, Kant’s Critique of Practical Reason and Other Works on the Theory of Ethics 
Kant’s Critique of Practical Reason and Other Works on the Theory of Ethics, trans. Thomas Kingsmill Abbott, B.D., Fellow and Tutor of Trinity College, Dublin, 4th revised ed. (London: Kongmans, Green and Co., 1889).
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of the propensity to evil in human nature.
By propensity (propensio) I understand the subjective source of possibility of an inclination (habitual desire, concupiscentia) so far as this latter is, as regards man generally, contingent. 1 (31) It is distinguished from a capacity by this, that although it may be innate, it need not be conceived as such, but may be regarded as acquired (when it is good), or (when it is bad) as drawn by the person on himself. Here, however, we are speaking only of the propensity to what is properly, i.e. morally bad, which, as it is possible only as a determination of free elective will, and this can be adjudged to be good or bad only by its maxims, must consist in the subjective ground of the possibility of a deviation of the maxims from the moral law, and if this propensity may be assumed as belonging to man universally (and therefore to the characteristics of his race) will be called a natural propensity of man to evil. We may add further that the capability or incapability of the elective will to adopt the moral law into its maxims or not, arising from natural propensity, is called a good or bad heart.
We may conceive three distinct degrees of this:—first, it is the weakness of the human heart in following adopted maxims generally, (32) or the frailty of human nature; secondly, the propensity to mingle non-moral motives with the moral (even when it is done with a good purpose and under maxims of good), that is impurity; thirdly, the propensity to adopt bad maxims, that is the depravity of human nature or of the human heart.
First, the frailty (fragilitas) of human nature is expressed even in the complaint of an apostle: “To will is present with me, but how to perform I find not;” that is, I adopt the good (the law) into the maxim of my elective will; but this, which, objectively in its ideal conception (in thesi) is an irresistible spring, is subjectively (in hypothesi), when the maxim is to be carried out, weaker than inclination.
Secondly, the impurity (impuritas, improbitas) of the human heart consists in this, that although the maxim is good in its object (the intended obedience to the law), and perhaps also powerful enough for practice, yet it is not purely moral, that is, does not, as ought to be the case, involve the law alone as its sufficient spring, but frequently (perhaps always) has need of other springs beside it, to determine the elective will to what duty demands. In other words, that dutiful actions are not done purely from duty.
Thirdly, the depravity (vitiositas, pravitas), or if it is preferred, the corruption (corruptio), of the human heart, is the propensity of the elective will to maxims which prefer other (not moral) springs to that which arises from the moral law. It may also be called the perversity (perversitas) of the human heart, because it reverses the moral order in respect of the springs of a free elective will; and although legally good actions may be consistent with this, the moral disposition is thereby corrupted in its root, and the man is therefore designated bad.
(33) It will be remarked that the propensity to evil in man is here ascribed even to the best (best in action), which must be the case if it is to be proved that the propensity to evil amongst men is universal, or what here signifies the same thing, that it is interwoven with human nature.
However, a man of good morals (bene moratus) and a morally good man (moraliter bonus) do not differ (or at least ought not to differ) as regards the agreement of their actions with the law; only that in the one these actions have not always the law for their sole and supreme spring; in the other it is invariably so. We may say of the former that he obeys the law in the letter (that is, as far as the act is concerned which the law commands), but of the latter, that he observes it in the spirit (the spirit of the moral law consists in this, that it is alone an adequate spring). Whatever is not done from this faith is sin (in the disposition of mind). For if other springs beside the law itself are necessary to determine the elective will to actions conforming to the law (ex. gr. desire of esteem, self-love in general, or even good-natured instinct, such as compassion), then it is a mere accident that they agree with the law, for they might just as well urge to its transgression. The maxim, then, the goodness of which is the measure of all moral worth in the person, is in this case opposed to the law, and while the man’s acts are all good, he is nevertheless bad.
The following explanation is necessary in order to define the conception of this propensity. Every propensity is either physical, that is, it appertains to man’s will as a physical being; or it is moral, that is, appertaining to his elective will as a moral being. In the first sense, there is no propensity to moral evil, for this must spring from freedom; (34) and a physical propensity (founded on sensible impulses) to any particular use of freedom, whether for good or evil, is a contradiction. A propensity to evil, then, can only attach to the elective will as a moral faculty. Now, nothing is morally bad (that is, capable of being imputed) but what is our own act. On the other hand, by the notion of a propensity we understand a subjective ground of determination of the elective will antecedent to any act, and which is consequently not itself an act. Hence there would be a contradiction in the notion of a mere propensity to evil, unless indeed this word “act” could be taken in two distinct senses, both reconcilable with the notion of freedom. Now the term “act” in general applies to that use of freedom by which the supreme maxim is adopted into one’s elective will (conformably or contrary to the law), as well as to that in which actions themselves (as to their matter, that is, the objects of the elective will) are performed in accordance with that maxim. The propensity to evil is an act in the former sense (peccatum originarium), and is at the same time the formal source of every act in the second sense, which in its matter violates the law and is called vice (peccatum derivativum); and the first fault remains, even though the second may be often avoided (from motives other than the law itself). The former is an intelligible act only cognizable by reason, apart from any condition of time; the latter sensible, empirical, given in time (factum phœnomenon). The former is especially called, in comparison with the second, a mere propensity; and innate, because it cannot be extirpated (since this would require that the supreme maxim should be good, whereas by virtue of that propensity itself it is supposed to be bad); (35) and especially because, although the corruption of our supreme maxim is our own act, we cannot assign any further cause for it, any more than for any fundamental attribute of our nature. What has just been said will show the reason why we have, at the beginning of this section, sought the three sources of moral evil simply in that which by laws of freedom affects the ultimate ground of our adopting or obeying this or that maxim, not in what affects the sensibility (as receptivity).