In his moral philosophy, Kant asks three basic questions: What can I know? What ought I to do? What may I hope for? The first question is theoretical, the second is practical, and the third, which raises religious issues, is both theoretical and practical, because it involves the concept of happiness in relation to worthiness and virtue. 
Central to Kant’s moral philosophy is the development of the notion of “radical evil” in human moral life and of the moral conversion that is needed to overcome it. The discussion of radical evil appears in book one of Religion Within the Boundaries of Mere Reason as a philosophical counterpart to the Christian doctrine of original sin (and his discussion of moral conversion in book two parallels the Christian doctrine of redemption), but unlike the classical notion of original sin, radical evil is self-incurred by each human being.
By using Christian parallels in his arguments, it seems that Kant’s purpose is not to salvage Christianity by transposing it into philosophical thinking; rather, Christian revelation helps him to deal with philosophical questions functioning as illustrations and ancillary concepts. 
Philosophers working within the enlightenment tradition disagreed on many issues, but one of the many similarities found in them was the belief in the power of reason to perfect the human being, in a way that his “salvation” comes, in a sense, from within, not from without (at least not from God, although perhaps from the community and its efforts). Enlightenment thinkers often tended to minimize ideas of sin and salvation, while stressing the importance of morality.
Kant takes the problem of moral evil very seriously, and describes human nature in ways dimmer than many of his predecessors; yet, while using Christian concepts for his arguments, there is a sense in which “salvation,” while necessary, is not the product of outside divine intervention of God in the human being – rather, it is the product of reason submitting to the moral law for its own sake.
What is, then, radical evil? To begin with, it is morally evil because it is an improper ordering of our maxims, so that the categorical imperative of the moral law is no longer the sole incentive for our choices and our actions.
Rather, we create other incentives – particularly self-love of different kinds – that take priority in the willing of our actions and thus become a propensity to evil fighting against the predisposition to the good. Overcoming radical evil requires a “change of heart” — i.e., a reordering of our fundamental principle of choice — that we are each responsible for effecting in ourselves.
For Kant, the ground of evil has to be intrinsically linked to the free exercise of choice. Evil is moral, and moral judgments only apply in the context of freedom. One is responsible for one has done freely, not for what he could not do otherwise. Any determination of nature or will would rule out moral responsibility, and thereby the adjectives good or evil could not apply to the agent.
Therefore, our natural inclinations are not the real problem; they are natural, and in some sense determined. They are driven either by the good principle of reason, or by an evil principle in the human heart. The real problem in human beings is to choose the right maxims.
The ground of evil has to be a rule that the power of choice produces for the exercise of its freedom, i.e., a maxim. A maxim is a universal rule one takes for himself, according to which he will to conduct himself. One’s free choice in ordering of the maxims (i.e., a guiding maxim to which other principles are subordinated in their own hierarchical order) is the ultimate ground that constitutes a person as good or evil; nothing could be asked any further as to the ground of the ordering of the maxims, for that would be another maxim, leading to an infinite regress. The first ground of the adoption of our maxims is the free power of choice.
Kant sees the human being as a unit guided by the maxims he has chosen, and particularly as they are ordered. Therefore, he rejects the common idea that the human being is partly good and partly evil, even if experience and observation seems to indicate that to be the case. That choice of maxims incorporates what will be the incentive which itself will the determine choices.
There are only two options then: either the incentive is the moral law itself or it is not.
- If the incentive is the moral law itself, the person is morally good;
- If it is not, he is morally evil.
Thus, one’s disposition concerning the moral law is never indifferent, for if the moral law is not the incentive, something else has taken its place.
This disposition is at the same time innate but it is also earned, because it is chosen (even if not chosen in time, and thereby not earned in time). To say that the human being is innately evil does not mean that he is determined to evil by nature, but rather that an evil maxim is posited in every use of his freedom.
For Kant, human beings have a predisposition to the good and a propensity to evil. A person develops as a person because he is made up of three parts: he is a living being (animality), a rational being (humanity) and a responsible being (personality). The last goes beyond his humanity because the moral aspect comes to bear.
There are then three kinds of predisposition to the good:
- First, the predisposition to animality, which is a mechanical self-love (for self-preservation, for the preservation of the species, and the social drive).
- Second, there is the predisposition to humanity, which involves a comparative self-love, i.e., the self-love that seeks happiness in the context of comparison and competition with other human beings.
- Lastly, there is the predisposition to personality, which is the “susceptibility to respect for the moral law as of itself a sufficient incentive to the power of choice.”
Human beings also have a propensity for evil, which exists in three degrees:
- First, there is the weakness of will in complying with maxims, which Kant calls frailty.
- Second, there is the propensity to mix immoral incentives with moral ones, which he calls impurity.
- Thirdly, there is the propensity to adopt evil maxims qua evil, which he calls depravity.
Thus, one can adopt the maxim, e.g., of never lying, and then fail to follow his maxim when tempted to do otherwise (frailty). Or one can always tell the truth because of the advantages that might bring, not solely because lying is morally evil (impurity). Or, one can rationalize that lying is the morally acceptable option given some particular circumstance (depravity).
Frailty is here illustrated with Paul’s assertion in Romans 7:15-16: “For I do not understand my own actions. For I do not do what I want, but I do the very thing I hate. Now if I do what I do not want, I agree with the law, that it is good.”
The propensity to impurity consists in the need of incentives other than the moral law itself for the will; one might do the right thing, but not purely from duty. The depravity, or perversity, is the reversing of the ethical order with regard to the incentives.
It is not What you do, but Why
Kant argues that the presence of evil can be taken for granted given the empirical observations and experiences of mankind; he knew from his ethnological studies that the human being is evil also in the “natural” state, as evidenced in gratuitous acts of cruelty.
However, Kant makes clear that what he is analyzing is not empirical actions, but the moral choices which have incorporated maxims behind them. One could be a person of good morals but not be a morally good human being, because the former can perform good actions (and refrain from bad actions) as a result of all kinds of incentives that are either mixed with or even substitutes for the moral law as the only incentive.
In this case, Kant argues (using another illustration from Scripture) that a human being of good morals complies with the law according to the letter but not according to the spirit. As he states,
[W]hatever incentives other than the law itself . . . are necessary to determine the power of choice to lawful actions, it is purely accidental that these actions agree with the law, for the incentives might equally well incite its violation. 
For this reason, empirical observation of deeds per se is inadequate to assess moral character; the issue is that which precedes every deed, i.e., the subjective determining ground of the power of choice. He realized that the actual seat of evil was not in the external actions but in the inner attitude of the human being. He deduced that evil must be an essential and universal condition and that its potential must lie in the human nature itself. In this sense, evil is radical.
In the next segment, we will address how Kant’s arguments necessarily reject an ethical code of human flourishing (eudaimonia) as the proper moral ground, and how he argues that ought implies can.
 Bernard M. G. Reardon, O 1975. “Kant as Theologian.” Downside Review. (1975; 93 (313)): 253.
 Philip Rossi, “Kant’s Philosophy of Religion”, The Stanford Encyclopedia of Philosophy (Fall 2005 Edition), Edward N. Zalta (ed.), URL = <http://plato.stanford.edu/archives/fall2005/entries/kant-religion/>.
 Peter Henrici, Wint 1991. “The Philosophers and Original Sin.” Communio. (18): 489.
 Immanuel Kant, George Di Giovanni, and Allen W. Wood.. Religion and Natural Theology. Cambridge edition of the works of Immanuel Kant (Cambridge University Press, 1996), 70-71.
 Ibid., 73
 Ibid., 76
 Ibid., 78.
 Peter Henrici, Wint 1991. The philosophers and original sin. Communio. (18): 495.
 Laurie McRobert,. “Kant and Radical Evil.” Fackenheim. (Toronto : Univ. of Toronto Pr., 1992.), 19.