Treatise on Sin and Vice

A. Vice and Sin 

Question 71: On vices and sins in themselves

Question 72: On the distinction among sins

Question 73: The relation of sins to one another

Question 74: The subject of sin

Question 75: The causes of sin in general

B. Original Sin and some other stuff 

Question 81: On the transmission of original sin

Question 82: On the essence of original sin

Question 83: On the subject of original sin

Brief notes on Questions 84-89

Question 71: On vices and sins in themselves
    71,1: Terminology: Virtue is by essence a certain habit by which one is disposed toward behaving in a way appropriate for perfecting his nature; hence, virtue is a certain sort of goodness that is directed toward good acts. Vice (vitium) is opposed to virtue directly insofar as a vice is a habit by which one is disposed to behave in a way inappropriate for perfecting his nature; badness (malitia) is opposed to virtue because it is directly opposed to the goodness that virtue involves; and sin (peccatum/culpa) is opposed to virtue insofar as it is directly opposed to the good act toward which virtue is ordered. 

    71,2: Vice is contrary to nature in the sense that by a vice one is disposed to act against that which agrees with and perfects nature, and hence because of vice one is subject to blame and censure. Since it is the rational soul that constitutes certain animals as human beings, vice in human beings is contrary to nature insofar as it is contrary to the order of reason.

Note ad 4: "The eternal law is related to the order of human reason as a craft is related to what is produced by it. Thus vice and sin are contrary to the order of human reason for the same reason that they are also contrary to the eternal law."
    71,3: Absolutely speaking, bad acts are worse than bad habits (vices), since actually doing evil is worse than being inclined to do evil. However (see ad 1), it is also true that bad habits are more enduring and lead to more bad acts, and in this sense they are worse than bad acts. 

    71,4: Some virtues, viz., the acquired virtues, can coexist with an opposed sin, even a mortal sin. For (i) the presence of a habit does not guarantee that one will use the habit on a given occasion, and (ii) acquired habits are neither generated nor destroyed by a single action. However, mortal sin is directly contrary to charity, which is the root of the theological virtues, and for this reason mortal sin is incompatible with charity and with all the other theological and infused virtues. (Faith and hope remain as inclinations, but not as virtues, strictly speaking.) The reason is that a mortal sin involves, either implicitly or explicitly, a contempt for God and a rejection of the redeeming love of Christ--and the infused virtues and gifts are precisely the effects of that which is rejected. 

    71,5: This article contains a discussion of sinful omissions and of whether such sins can occur without any act and, more specifically, without any sinful act. St. Thomas replies that they can sometimes occur without any act, either interior or exterior. For even though one can explicitly will a certain omission (e.g., willing not to go to church on Sunday), this is not necessary. For instance, I might omit to go to church on Sunday without ever having so much as thought about going or not going. This can still be a sin even if it does not directly involve any act. However, if we look at the causes of omissions, we will always find some act or acts which lead to the omission, even if the omission itself does not itself involve any act. Sometimes the causes of an omission might not fall within the will's power, and in such cases they render the omission involuntary and hence not a sin. However, when the causes are subject to the will, then there must always be at least an interior act which is a cause of the omission. Yet even here we have to draw a distinction. Sometimes the interior act in question leads directly to the omission itself, as when I will not to go to church because going to church takes too much effort. Sometimes the causing act is only incidentally related to the omission, as when I wish to play golf at a time at which I should be going to church or as when I stay up too late to watch a movie and oversleep--without ever having taken into account that I should go to church. In such cases we should say that there is a sin without any act directly involved in it. 

    71,6: This is an important article for helping us to understand the relation of good and evil to the ultimate end, on the one hand, and to eternal and natural law, on the other. St. Thomas says that, strictly speaking, a sinful act is bad by virtue of not conforming to eternal law and (derivatively) by virtue of not conforming to the order of reason. In answer to the objection that if that were so, then sins would be bad because prohibited rather than the other way around, but that in fact they are bad primarily because they are not ordered to our ultimate end, he replies that it is the eternal law that principally and primarily orders us toward our end. So we see here the coming together of an ethics according to which good and evil are defined in terms of conformity (or lack of conformity) to the ultimate end, and of an ethics according to which good and evil are defined in terms of obedience (or disobedience) to law. On St. Thomas's view the two necessarily go together, since in creating human beings God necessarily legislates for them in such a way that obedience to the law leads them to happiness as defined by their natures.  This is because God is a loving legislator who cares for the common good--the common good of the universe as a whole (eternal law in general) and the common good of human beings (eternal law as the source of natural law).

Question 72: On the distinction among sins  

72,1: Two things are involved in a sin: (i) a voluntary act which the sinner intends in such-and-such matter, and (ii) the act's disorderedness, which consists in its departure from God's law. (i) is per se and directly intended, while (ii) is related per accidens to the sinner's intention. That is, the sinner directly wills to do X, which is a sin; so that there is always some X, distinct from sinning, which is the direct object of the sinner's intention. The distinction among sins thus follows the distinction among voluntary acts, and such acts are distinct from one another according to their objects. 
    Compare this with the pear tree incident in book 2 of Augustine's Confessions.  There Augustine worries that his motive in stealing the pears was simply to do something sinful or lawless. This would not, as far as I can tell, contradict the first point St. Thomas is making here, since Augustine directly intended to steal the pears.  However, Augustine tries mightily to avoid the conclusion that what he really intended in stealing the pears was to do something lawless. If that were true or possible, then it seems that he would be directly intending both the act of stealing and its lawlessness. 

    72,2: "Every sin consists in the desire for some mutable good which is desired inordinately and, as a result, one takes a disordered pleasure in such a good when it is possessed." So sins are divided into carnal and spiritual according to the mutable goods desired and thus according to whether their possession gives mental (spiritual) pleasure (as with sins of pride, envy, avarice, sloth, and anger) or corporeal pleasure (as with sins of lust and gluttony). 

    72,3: Sins are not distinguished in species by their efficient or final causes, i.e., by the active principles (e.g., a passion) that lead to them or by what the agent's non-proximate ends are. On the other hand, if we're trying to root out our sins, then whatever species they might belong to, it's crucial to root out their causes. For instance, if I habitually steal in order not to have to work, then my sin is rooted in sloth even though it is a sin of avarice. In virtue terms, it is a sin against justice that is rooted in a sin against temperance 

    72,4: Sins can also be classified as sins against God, against oneself, and against one's neighbor, depending on their per se objects. Sins against God are opposed to the theological virtues; sins against oneself are opposed to the virtues of temperance and fortitude; and sins against one's neighbor are opposed to the virtue of justice. 

    72,5: Sins are not divided into species by the degrees of guilt and susceptibility to punishment (reatus poenae) which accompanies them, i.e., by whether they are venial or mortal. Rather, this distinction is posterior to the division of sins according to species. The difference between venial and mortal sin is analogous to the difference between sickness and death. Thus, the disorderedness in a sin can consist in a total aversion from the ultimate end or instead in a deviation that does not constitute a total aversion. "Hence, nothing prevents its being the case that mortal and venial sins can be found within the same species of sin. For instance, the first movement in the genus of adultery is a venial sin; and an idle word, which is usually a venial sin, can also be a mortal sin. 

    72,6: Again, the difference between sins of commission and sins of omission follows upon the division of sins according to species. 

      Note ad 2: "Different negative and affirmative precepts had to be proposed in God's law so that men might be introduced to virtue gradually--first by abstaining from evil, which we are led to by the negative precepts, and then by doing good, which we are led to by the affirmative precepts."

    72,7-9: The traditional distinction of sins of the heart, sins of the mouth, and sins of the deed is best construed as depicting the evolving grades of a complete sin. The beginning of sin is in the heart as in a foundation (this involves three stages: the thought, enjoying the thought, consenting to the thought); the second grade is in the mouth, i.e., the verbal manifestation of what is in the heart; the third grade is the accomplishment of what is in the heart through the act or deed. Excess and defect are indicative of sins of different species in that they are motivated by love (sins of excess) and hate (sins of defect). And as with acts in general, sometimes circumstances alter the species of sin because they are included in the species of sin.

Question 73: The relation of sins to one another  

    73,1: It is not the case that all sins are connected, since some sins are contrary to others. Unlike the virtues, which reflect the unity of a virtuous life lived according to a self-consistent rule of reason and of God's law, the vices are disintegrative. In ad 2, St. Thomas says some illuminating things about the relation of sin to virtue: 
      "It is not the case that just any sinful act destroys the opposed virtue. For a venial sin does not destroy a virtue. And while a mortal sin destroys an infused virtue by turning away from God, no one act of sin, even mortal sin, destroys the habit of an acquired virtue. However, if the acts are repeated up to the point that a contrary habit is generated, then the habit of the acquired virtue is destroyed. But once this habit is destroyed, prudence is destroyed, since in acting against any virtue, a man acts against prudence. But no moral virtue can exist without prudence ... And so, as a result, all the moral virtues are excluded as regards the perfect and formal being of a virtue that such virtues have insofar as they participate in prudence. Still, there remain inclinations to the acts of the virtues, though such inclinations do not themselves have the character of a virtue." 

      This is from ad 3: "Love of God is integrative, since it reduces a man's affective state from a multiplicity to a unity; and this is why the virtues, which are caused by love of God, are connected. But love of self splits a man's affective state into diverse elements, since a man loves himself by desiring for himself temporal goods, which are diverse and varied; and this is why vices and sins, which are caused by love of self, are not connected."   

    73,2-4: It is not the case that all sins are equal. For some privations, e.g., sickness, admit of degrees, since deviations from the norm are possible that do not constitute a complete undermining of the relevant principle--which in the case of sickness is an appropriate ordering of the humors and which in the case of sin is the order of reason. And just as one sickness (e.g., heart disease) is worse than another (say, a cold), so too one sin is worse than another to the extent that the principle that it attacks is prior according to the order of reason. In general, the seriousness of a sin varies with the gravity of its object or end (proximate and ultimate). For example, sins with respect to the substance of man (e.g., homicide) are more grave than sins with respect to exterior goods (e.g., theft), and both are less grave than sins that are immediately opposed to God (e.g., infidelity, blasphemy, etc.). Notice that every sin involves both a turning toward (conversio) some mutable good and a turning away (aversio) to some degree from the immutable good. It is the latter that makes them evil, whereas the degree of seriousness has to do with the object. So a sin with respect to God is more serious, ceteris paribus, than a sin against man, and the latter is more serious, ceteris paribus, than a sin with respect to exterior goods. And within each of these three orders, a sin is more or less serious to the extent that it has do with something more or less central or important. Furthermore, speaking per se, the greater a virtue is in itself, the more serious are sins against that virtue. On the other hand, there is a sense in which the greater the virtue, the less serious are the sins opposed to it. For to the extent that one's possession of a virtue is stronger or more intense, to that extent the virtue helps keep one from committing even small sins against the virtue. 

    73,5-6: Ceteris paribus, carnal sins involve less guilt or blameworthiness than spiritual sins, and this for three reasons. First, the subject of a carnal sin has more of the character of a turning toward a mutable good (sense pleasure), whereas the subject of a spiritual sin has more of the character of a turning away from God; but the latter is the basis of the guilt or blameworthiness that a sin carries with it. Second, the matter (materia circa quam) of a carnal sin is one's own body, whereas the matter of a spiritual sin is God and one's neighbor, which are to be loved more than one's own body. Third, the (prevolitional) impulse toward carnal sin is greater than the impulse toward spiritual sin; but there is less guilt involved in a sin to the extent that the impulse toward it is greater. This last point is important. There are two sorts of causes of sin. The first, which is the per se cause, is the very act of willing the sin, which has the sin as its fruit. So the more intense the willing to sin is, the graver the sin. The second are the causes of sin exterior to the will. Some of these, like the object willed, induce the will to sin with respect to the very nature of the will, e.g., the end. And such exterior causes augment the sin; the worse the end, the worse the sin. Others incline the will to sin but in a way that is outside its nature. They do so by diminishing either (i) the judgment of reason (e.g., ignorance, antecedent passions such as anger and concupiscence) or (ii) the will's free movement (weakness, violence, fear). 

    73,7-10: The remaining articles have to do with the question of how circumstances of various types affect the gravity of a sin. Briefly, St. Thomas argues that (i) circumstances can make a sin graver either (a) by changing the genus of the sin (e.g., an act that would otherwise be fornication becomes a graver sin, viz., adultery, when it is committed with the spouse of another) or (b) by adding another independent dimension of disorder to the sin (giving money away not only when one ought not to but to someone whom one ought not to) or (c) by augmenting some already existent disorder (stealing a lot of money from someone); that (ii) intended and/or foreseen harm, and sometimes even unintended and unforeseen harm, inflicted as a result of the commission of a sin, makes the sin more serious; that (iii) the status of the person against whom one sins may aggravate the sin (in light of his office or of his virtue); that (iv) the status of the sinner himself may aggravate the sin--for instance, the more virtuous the person, the worse the sin if the sin proceeds from deliberation rather than from a sudden impulse that the person gives in to because of human weakness. 

      Interesting case: A believer who commits what he knows to be a sin subject to a greater punishment commits a greater sin simply because he persists in what he knows to be a more serious offense against God, whereas a non-believer might commit the same sin without knowing of the punishment for it and hence without disdaining God as much.

Question 74: The subject of sin

    74,1-2: The will is the principal subject of every sin, but is not the only subject of sin. This is easy to see from what St. Thomas has earlier said about the relations among the will, the intellect, and the sentient appetite. 

    74,3-4: The movements of sensuality (i.e., the sentient appetite) are subject to the control of reason, and so the sentient appetite can be a subject of sin, though it cannot in itself be the subject of mortal sin. Of course, acts of the sentient appetite can dispose one to commit a mortal sin, which can have only the rational faculties as its subject. The same holds for virtuous actions; they never belong to the sentient appetite alone, since every act of a moral virtue is accompanied by an act of prudence. 

      (Note St. Thomas's claim--in art. 3, ad 2--that even though, because of the fomes peccati, we are incapable of thwarting all the inordinate movements of the sentient appetite in this life, nonetheless, with respect to each such movement we are able to thwart that one. Compare: no pro basketball player is capable of making all his free throws throughout an entire season, but with respect to each such free throw, he is able to make that one.)

    74,5-6: Reason is the subject of sin in two ways: (i) when it does not know, or is mistaken about, a truth it should know, and (ii) when it commands or does not restrain inordinate acts of the lower powers. Now reason sins not only by deliberately commanding inordinate movements of the passions, but also by deliberately failing to advert to or to repel inordinate movements of the passions, e.g., when it deliberately fails to advert to or repel an illicit pleasure and allows it to be prolonged (delectatio morosa).

    74,7-10: St. Thomas here discusses consent and, in so doing, introduces Augustine's distinction between higher and lower reason. Higher reason considers, judges, and rules in the light of eternal reasons, i.e., divine law, whereas lower reason considers, judges, and rules in the light of temporal reasons. In general, sinful consent is attributed to reason (either the higher reason or the lower reason). It can sometimes be the case that consent to pleasure is itself a mortal sin, as when one deliberately consents to the pleasure he feels while thinking about an illicit object or act of a lower power which is itself mortally sinful. On the other hand, higher reason can sin venially, and not mortally, even when it deliberately consents to a venial sin. For in doing so it does not necessarily act out of contempt for the law of God.

Question 75: The causes of sin in general

    75,1: A sin is an inordinate act. As an act, it must have one or more per se causes, just like any other act. But the more interesting question has to do with its defectiveness. And this defectiveness is traced back to the will's lack of conformity to right reason and to the law of God. So the act is caused by the will (as well as by God as a concurring cause), and the fact that the act thus caused is defective is traced back to the will (and not to God)--the will, which, lacking the direction of reason, intends some mutable good in preference to the immutable Good. 

    75,2: The immediate interior causes of sin are reason and will; the mediate interior causes are sentient apprehension (or imagination) and appetite. Reason is a cause of sin to the extent that sin involves the absence of an appropriate motive, viz., the rule of reason or of divine law; the will is a cause of sin insofar as it is the will which brings a voluntary act to perfection. The sentient powers are a cause of sin because when a sensible good is proposed as desirable and the sentient appetite is inclined toward it, reason sometimes ceases to consider the appropriate rule. 

    75,3: Corresponding to the interior causes of sin there are three imaginable exterior causes of sin. The first would be an exterior cause that directly moves the will; but only God can move the will from within and God cannot be a cause of sin. So no exterior agent causes a sin by moving the will. The second sort of exterior cause moves reason through persuasion of one sort or another. It is in this way that demons and other human beings can move us to sin. The third sort of exterior cause is any object that might move the sentient appetite. But in these two latter cases there is no necessary connection between the action of the causes involved and the commission of the sin, since it is always in the will's power to resist the persuasion or the attractiveness of the object. 

    75,4: Sin can be a cause of sin in any of the four categories of cause:   

      Efficient cause:

        a. per accidens: One sin can remove an obstacle to some other sin; for example, if I lose grace or charity or shame through one sin, it is easier to fall into some other sin. 

        b. per se: One sin can dispose the person to sins of the same sort through habituation. 

      Material cause: By one sin I can acquire goods (e.g., money) which put me in position for another sin that presupposes those goods (e.g., disputes over money). 

      Final/formal cause: I might commit one sin for the sake of committing another; e.g., I might render an unjust judgment for the sake of committing adultery. In such a case the adultery is, as it were, the final as well as, in some sense, the formal cause of the unjust judgment.

Question 81: On the transmission of original sin

    81,1: Interestingly, it is only in this tract on original sin that we come to realize how bad off our starting position is in relation to the attainment of happiness. (Question: Why didn't St. Thomas tell us this at the beginning, in the Treatise on Happiness?) The condition of original sin (to be described below) is transmitted to the descendants of Adam through their origin. For the very nature of a living human being--that which is transmitted to us in our generation from our parents--is itself fallen from the state of original justice. This sin or aberration is imputed to us not as personal sin, but insofar as we are "members" of Adam through our sharing in his life and nature through natural generation--just as a sin committed with one's hand is imputed to the hand not in itself, but only insofar as it is a member of the human being who sins.  In like manner, the redemption and spiritual restoration of our nature is imputed to us not as personal merit, but only insofar as we are "members" of Christ through our sharing in his life and nature through spiritual regeneration (baptism). [Note how these doctrines offend our individualistic sensitivities, and also note the parallel between them.] 

    81,2-3: The reason that the first sin of Adam, and not his other sins, is transmitted to his posterity is that the first sin, unlike the others, has an effect on human nature itself as it was given by God to Adam. For God gave Adam a nature which not only had its own intrinsic principles but also had the gift of grace called original justice, which God meant to be transmitted to Adam's posterity through generation. This is why we talk of "fallen nature": it is human nature existing with its own principles but lacking a supernatural gift (with its preternatural overflow) by which it had been elevated and which God had intended to be transmitted to Adam's posterity through generation. And it is because this fallen nature is transmitted to us that we stand in need of the healing and redemption which is made possible for us only through the merits of Christ. (Note, however, that those who have been re-generated through baptism do not pass on a re-generated nature; for baptism is a spiritual re-generation which does not restore all the gifts lost through Adam's sin--see below.) 

    81,4-5: In yet another assault on our individualistic proclivities, St. Thomas asserts that if God were to miraculously create a human being who is not derived in origin from Adam, that human being would not have a fallen nature. Also, St. Thomas holds that our nature would not have been corrupted just by Eve's sin if Adam had not sinned. (Note ad 3 for a counter to those who misunderstand part of the rationale behind St. Thomas's rejection of the Immaculate Conception of the Mother of God.)

Question 82: On the essence of original sin

    82,1-4: Original sin is a "habit" in the sense in which health and sickness are habits, i.e., dispositions of a nature composed of many elements according to which one is related well or badly to a standard set by the nature. The condition of original sin is had by us through our origin rather than through our actions

    So original sin has two sides: 

      a. negative: original sin is a privation, derived from the sin of Adam, of the harmonious state of original justice, in which (i) all of the passions were ruled by reason, (ii) intellect and will were subject to God's law, and (iii) man was endowed with grace and justified in the sight of God, with (a) all the theological and moral virtues along with (b) freedom from sickness and death. (The total subjection of the passions to reason and the freedom from sickness and death are called preternatural gifts and are not restored through baptism; the indwelling of God through grace, along with the theological and infused virtues, are supernatural gifts which are restored through baptism.)

      b. positive: as a result of this privation of original justice, which had kept the powers of the soul ordered in their movements, all the passions and powers of the soul are left free, as it were, to go their own way, resulting in the "dis-integration" or "dis-ordering" of the powers of the soul. The fact that the particular effects may vary in strength from person to person has to do with the particular "matter" informed by the soul.   

    In summary, original sin is formally the absence of original justice caused by the turning away of the will from God; this lack of original justice results materially in the disordering of the powers of the soul--a disordering that can be given the general name of concupiscence

    For more on the effects of original sin on the powers of the soul, see 85,3: "Through original justice reason perfectly controlled the lower powers of the soul, and reason itself was perfected by God as subject to Him. But this original justice was lost through the sin of the first parent ... And so all the powers of the soul remain in some way destitute of the proper order by which they are naturally ordered to virtue--and this destitution itself is called the wounding of the nature. Now there are four powers of the soul that can be the subject of virtues ... viz., reason, in which prudence resides; the will, in which justice resides; the irascible part, in which fortitude resides; and the concupiscible part, in which temperance resides. Insofar as reason is stripped of its own ordering toward truth, there is the wound of ignorance; insofar as the will is stripped of its ordering toward the good, there is the wound of malice; insofar as the irascible part is stripped of its ordering toward the arduous, there is the wound of weakness; and insofar as the concupiscible part is stripped of its ordering toward the pleasurable as moderated by reason, there is the wound of concupiscence."

Question 83: On the subject of original sin

    83,1-2: Original sin has the soul as its immediate subject, since only the soul can bear the character of guilt or fault that accompanies original sin. Furthermore, it is the soul's essence, rather than the powers that emanate from that essence, that is the immediate subject of original sin, since original sin affects the very nature that is transmitted from Adam to his descendants. (Likewise, we will later (110,4) see that habitual grace has the essence of the soul, and not the powers, as its immediate subject, since it effects a change in the very essence of the soul that then redounds upon the soul's powers, which receive the infused theological and moral virtues.) 

    83,3: Insofar as it affects the powers of the soul, original sin infects the will prior to the other powers, since among those powers the will has the first inclination toward sinning. And it especially infects the powers that contribute to generation, viz., the generative power, the concupiscible power, and the sense of touch.

Notes on Questions 84-89:

    Question 84: Cupidity (or avarice) is the root of all sins, since (i) cupidity in the broad sense includes any disordered inclination toward a conversion to corruptible goods, thus serving as the "root" which furnishes nourishment for the "tree" of sin; and since (ii) cupidity in the narrow sense (desire for wealth) is the "root" which supplies the material wherewithal by which the "tree" of sin might flourish.  Pride (superbia), on the other hand is the beginning of all sins, since (i) pride in the broad sense, as an inclination toward contempt of God, is the source of the aversion by which we turn away from God and subjection to his law, whereas (ii) pride in the narrow sense of an inordinate desire for one's own excellence serves as the ultimate motive or end in the order of intention for inordinate appetites which are "fed" or facilitated by wealth. 

    The seven capital sins (vainglory, gluttony, lust, avarice, sloth, envy, and anger) are principles of other sins and are "directive of and conducive to" other sins in the sense that they provide ends to which other sins are means. St. Thomas orders them according to the goods they are turned toward and the evils they flee from: 

    Good to be pursued:
    Goods of soul:  Praiseworthiness
    Vainglory (inanis gloria)
    Goods of body:  Conservation of body 
    Gluttony (gula)
    Goods of body:  Conservation of species
    Lust (luxuria)
    External goods: Wealth
    Greed (avaritia)
    Evil to be avoided:
    Hard work to get intellectual or spiritual goods
    Sloth (acedia)
    Good of another perceived as evil for oneself
    Envy (invidia)
    Another's good perceived as evil for oneself because of an urge for vengeance
    Anger (ira)

    Question 85: What are the effects of sin? (See 109,7 for summary.) First of all, sin diminishes the goodness of the nature (corruptio boni naturae) of the sinner in the sense that it diminishes our natural inclination toward virtue; and it does this by posing impediments to the fulfillment of that inclination. So the inclination itself is always there, but because of sin it becomes harder and harder for that inclination to achieve its end.  In particular sin intensifies the wounds of nature that begin with original sin:  (i) intellect: ignorance; (ii) will: malice; (iii) irascible appetite: weakness;  (iv) concupiscible appetite: concupiscence.  These wounds remain to certain degrees even in the wake of forgiveness. 

    Question 86: The second effect of sin is the blemish or stain of sin (macula peccati), which darkens the brightness (nitor) of both the light of natural reason and the divine light of wisdom and grace. When we love something ordinately, the contact our soul has with that thing (or person) brightens it, as it were.  By contrast, when we love something inordinately--that is, contrary to the light of reason, wisdom, and faith--the contact our soul has with that thing (or person) darkens or blemishes the soul.  (Sometimes olfactory metaphors are used here, as when spiritual writers talk of the "good odor" of sanctity.)  Thus one who is in the state of mortal sin is a child of darkness. The stain or blemish of sin remains until the will, by a contrary motion prompted by actual grace, reverts through grace to the light of reason and of the divine law.  Thus, when a sin is forgiven, it is this blemish that is removed. 

    An important corollary is this:  As long as the stain of sin remains, the sinner is cleaving inordinately to mutable goods and thus his desire is to have those things.  In this state he does not desire union with God.  This is the key to understanding eternal punishment.  The reprobate do not want the punishment that is due sin, but neither do they want to come face to face with infinite goodness.  Just the opposite, they abhor the light of glory and with it clear witnesses of that light.  This helps to explain the depth of the hatred Jesus' enemies had for him. 

    Question 87: A third effect of sin is the sinner's being deserving of punishment (reatus poenae). All sin, whether mortal or venial, incurs this punishability, since all sin causes an imbalance (or, alternatively, a dent) in the order of reason, or the order of human governance, or the order of divine rule; and it is only through punishment that this order is restored. As long as the blemish of sin (in the case of mortal sin) remains, the punishment as such is in every way contrary to the sinner's will and has the full character of punishment.  Note that (art. 2) sin itself can be the punishment for sin, given that by withdrawing his grace God allows us to be "given up to our own desires."  

    If the sin is only venial, or if the blemish of sin has been removed through the mortal sinner's repentance, then the punishment is "satisfying punishment" (poena satisfactoria), which the sinner freely takes upon himself. Notice that such satisfying punishment can also be taken upon oneself for the sins of others. Indeed, this is precisely the nature of Christ's "punishment".

    Questions 88-89: A venial sin is reparable internally (like a sickness which the body itself repairs), because it does not undermine the first principle of moral and spiritual life, which is an ordering to the true ultimate end. A mortal sin is, by contrast, irreparable internally. Only God by his grace can offer hope for a remedy. In De Malo 7,1 St. Thomas compares mortal sin to a terminal disease and venial sin to a disease that is not life threatening and which can be overcome by the body's life-principle. If we have time I will say more about this article from the De Malo.