1. The first principle of morality (see 7‑F) makes it clear that moral evil is privation in the existential domain of openness to integral human fulfillment. Sin really is the same thing as moral evil, but the concept of sin considers moral evil under a certain aspect.4 Sin is moral evil considered precisely insofar as it is contrary to the good of religion—contrary, that is, to the fulfillment of humankind’s potential for harmony with God. Moral evil blocks human fulfillment on every level of existence (see GS 13), disintegrating the self, disrupting personal life, dismembering community, and distorting the relationship of humankind with God. Thus every morally evil act offends God, and sin is immorality considered under this specific aspect (see S.t., 1–2, q. 71, a. 6, ad 5).
2. A person who chooses according to a nonrational principle of self-determination need not intend to offend God (see S.t., 1–2, q. 72, a. 1; q. 73, a. 1; q. 75, aa. 1–2; q. 78, a. 1). Perhaps one wishes only to enjoy oneself (see S.t., 1–2, q. 71, a. 2, ad 3; q. 77). Even so, moral evil cannot be without sin (see C below for further clarification).
3. Moral evil is privation in the existential domain. One might say that moral evil precisely is in the free choice which is not as it should be (see S.t., 1–2, q. 79, a. 2). St. Augustine sometimes speaks this way (see FEF 1558 and 1560). More broadly, sin not only is in free choice but in all the varieties of voluntariness, to the extent that willing is not as it should be (see S.t., 1–2, q. 71, a. 5). Sometimes sinful willing affects and involves others besides those who sin by their own choice. Hence, as Augustine teaches, one person can be in sin by another’s choice (see FEF 1454). Such is the case with original sin, to be treated in chapter fourteen.
4. Prior to free choice one understands the human goods and spontaneously wills them, but one’s will is not determined to respect all of them fully in every instance. In making an evil choice, one responds to some principles of practical reason but not all; one determines oneself in regard to some good but fails to determine oneself in a manner consistent with openness to integral human fulfillment. The privation which constitutes the evil of immorality is precisely this lack of reasonableness and openness which could and ought to be present in every free choice. To put this another way, one’s choice could and ought to conform perfectly to God’s law made known through one’s conscience; a morally evil choice does not; this lack of due conformity is the privation which constitutes the moral evil of the act (see S.t., 1–2, q. 71, a. 6; q. 79, a. 2).
5. The privation is the evil. A sin centrally is a choice subject to this evil. The privation is not imposed on choice from without. Rather, it is freely accepted in the free choice by the free choice itself (see FEF 1549). A sin is therefore something real (in the primary instance, it is a choice); it is a sin because of something real, namely, a real privation of right order. Yet this privation is not a positive reality; it is not something which needs a causal explanation, as if it were a distinct creature. For this reason, sin as such in no way depends on God (see S.t., 1–2, q. 79, a. 2; S.c.g., 3, 162). Instead its source is altogether one’s own bad will (see Jas 1.13–15).
Evil will and good will are not counterparts. Only the good has positive reality from God. Augustine explains: “Let no one, therefore, seek the efficient cause of an evil will; it is not efficient but deficient, because the will in this case is not an effecting of something but a defecting. To defect from that which supremely exists to that which has less being is to begin to have an evil will. But to try to find causes of those defections, since, as I said, they are not efficient but deficient, is as if someone tried to see darkness or to hear silence” (FEF 1754).
6. Insofar as a choice is morally evil, the behavior which carries it out shares in moral evil. Thus the sinful deed, the deviant behavior, has the character of moral evil not from its positive reality but from the sinfulness of the choice it executes. Sin is from the heart (see S.t., 1–2, q. 74, aa. 1–2). And the choice itself is not evil by what it positively is but by the privation one accepts in making it. Sin is unnecessary, unreasonable self-limitation and self-mutilation (see S.t., 1–2, q. 29, a. 4; 2–2, q. 25, a. 7).
For this reason, it is a mistake to base moral criticism and exhortation on any positive qualities of sinful behavior. Sexual immorality, for example, is not sinful because it is ugly, or because it can lead to disease, or because it might end in unwanted pregnancy. Murder is not sinful because it gets blood on the floor. Criticism and exhortation based on such grounds distract attention from real moral concerns, and encourage people to find more acceptable ways of sinning—for example, sexual indulgence which is not ugly, unhealthy, or unwantedly fruitful, and murder which is bloodless.
7. Sin primarily is in sinful choices, and sinful choices are of themselves spiritual entities which persist. Hence, as St. Augustine notes (see FEF 1873), the guilt of sin—which is sin itself—lasts even after the sinful behavior is past and forgotten. Anyone who understands what immorality is knows that no sin is a mere passing event or temporal process. In every free choice, one makes oneself be what one is by choosing. In every sinful choice, one makes oneself guilty (see S.t., 1–2, q. 86; q. 87, a. 6). There is no need to think of a state of sin, a habit of sin, or a condition of guilt distinct from the sin itself. Sin is of itself a state, a habit, a condition of guilt.
8. Moral evil spreads from sinful choices throughout the other forms of voluntariness. Gradually but inevitably, it pervades and perverts the whole existential domain of personal and interpersonal relationships, engendering disharmony and alienation on every level (see S.t., 1–2, q. 85, aa. 1–3; q. 87, a. 1). From the existential domain, the perversity which begins in free choice introduces disorder even into the other dimensions of individual and social reality. It leads to disease and death, confusion and error, shoddiness and breakdown.
9. The expressions “formal sin” and “material sin” mark a useful distinction. Any choice which is not what it morally ought to be can be called “sin,” but sometimes such a choice does not violate conscience because one is unable to make a judgment of conscience or one’s conscience is in error. A choice which conforms to an erring conscience or a wrong choice made without moral reflection is called a “material” sin, while one which violates conscience is called a “formal” sin. Those who make choices but are simply unable to make relevant judgments of conscience are blameless—for example, children and persons who are mentally ill can lack the capacity to discern moral truth, at least in some matters. One is not always blameless in following an erring conscience, for the error can be one’s own fault (see 3‑B). Material sins which are in conformity with a culpably erroneous conscience are thus not free of guilt.
4. Some scholars who write on sin lack insight into any nonlegalistic concept of moral evil. They have no understanding of authentic natural-law theory. Hence, they think that a “moralistic” conception of sin is mere legalism—deviation from externally imposed norms. To this they contrast a theological, “personalistic” conception—personal alienation from God. See, for example, S. J. de Vries, “Sin, Sinners,” Interpreter’s Dictionary of the Bible, 4:362–63, where such a defect mars an otherwise helpful article. To some extent, to use responsibility, conceived as answerability to God, as the basic category of religious ethics is a sign of the same defect. See, for instance, Charles E. Curran, “Responsibility in Moral Theology: Centrality, Foundations, and Implications for Ecclesiology,” The Jurist, 31 (1971), 113–42, where the responsibility motif is advanced in contrast to the institutional motif, in a framework where the moral teaching of the Church is misconceived in a thoroughly legalistic way.