Thomas Aquinas

“ If the consent to the evil use of things that can be perceived by the bodily senses, so far approves of any sin, as to point, if possible, to its consummation by deed, we are to understand that the woman has offered the forbidden fruit to her husband. ”

“ Now it happens that an action, just as a thing, is an object of delectation, because the action itself can be considered as a good and an end, in which the person who delights in it, rests. Sometimes the action itself, which results in delectation, is the object of delectation, in so far as the appetitive power, to which it belongs to take delight in anything, is brought to bear on the action itself as a good: for instance, when a man thinks and delights in his thought, in so far as his thought pleases him ”

“ The act of the sensuality can concur towards a mortal sin: yet the fact of its being a mortal sin is due, not to its being an act of the sensuality, but to its being an act of reason, to whom the ordering to the end belongs. ”

“ Disease of the heart is twofold: one which is in the very substance of the heart, and affects its natural consistency, and such a disease is always mortal: the other is a disease of the heart consisting in some disorder either of the movement or of the parts surrounding the heart, and such a disease is not always mortal. In like manner there is mortal sin in the higher reason whenever the order itself of the higher reason to its proper object which is the eternal law, is destroyed; but when the disorder leaves this untouched, the sin is not mortal but venial. ”

“ Man does not do perfectly himself what he does without the deliberation of reason, since the principal part of man does nothing therein: wherefore such is not perfectly a human act; and consequently it cannot be a perfect act of virtue or of sin, but is something imperfect of that kind. Therefore such movement of the sensuality as forestalls the reason, is a venial sin, which is something imperfect in the genus of sin.
Art. 4 - Whether mortal sin can be in the sensuality? Objection 1: It would seem that mortal sin can be in the sensuality. Because an act is discerned by its object. ”

“ If, however, the defect of reason be about something which a man is able and ought to know, he is not altogether excused from sin, and the defect is imputed to him as a sin. The defect which belongs only to the act of directing the other powers, is always imputed to reason as a sin, because it can always obviate this defect by means of its proper act. ”

“ But that a man in thinking of fornication takes pleasure in the act thought of, is due to his desire being inclined to this act. Wherefore the fact that a man consents to such a delectation, amounts to nothing less than a consent to the inclination of his appetite to fornication: for no man takes pleasure except in that which is in conformity with his appetite. ”

“ A disposition may be related in three ways to that to which it disposes: for sometimes it is the same thing and is in the same subject; thus inchoate science is a disposition to perfect science: sometimes it is in the same subject, but is not the same thing; thus heat is a disposition to the form of fire: sometimes it is neither the same thing, nor in the same subject, as in those things which are subordinate to one another in such a way that we can arrive at one through the other, e.g. goodness of the imagination is a disposition to science which is in the intellect. ”

“ The continual corruption of the sensuality is to be understood as referring to the "fomes," which is never completely destroyed in this life, since, though the stain of original sin passes, its effect remains. However, this corruption of the "fomes" does not hinder man from using his rational will to check individual inordinate movements, if he be presentient to them, for instance by turning his thoughts to other things. ”

“ Therefore not only the will can be a subject of sin, but also all those powers which can be moved to their acts, or restrained from their acts, by the will; and these same powers are the subjects of good and evil moral habits, because act and habit belong to the same subject.
Reply to Objection 1: We do not sin except by the will as first mover; but we sin by the other powers as moved by the will. ”

“ But sometimes consent is given to an act, without consulting the eternal law: since man does not always think about Divine things, whenever he consents to an act. Therefore the sin of consent to the act is not always in the higher reason. ”

“ Yet while he is turning his thoughts to something else, an inordinate movement may arise about this also: thus when a man, in order to avoid the movements of concupiscence, turns his thoughts away from carnal pleasures, to the considerations of science, sometimes an unpremeditated movement of vainglory will arise. Consequently, a man cannot avoid all such movements, on account of the aforesaid corruption: but it is enough, for the conditions of a voluntary sin, that he be able to avoid each single one. ”

“ It would seem that consent to delectation is not a mortal sin, for consent to delectation belongs to the lower reason, which does not consider the eternal types, i.e. the eternal law, and consequently does not turn away from them. ”

“ One and the same thing may be the subject of different considerations, of which one is higher than the other; thus the existence of God may be considered, either as possible to be known by the human reason, or as delivered to us by Divine revelation, which is a higher consideration. ”

“ If the defect in the apprehensive power were nowise subject to the will, there would be no sin, either in the will, or in the apprehensive power, as in the case of those whose ignorance is invincible. It remains therefore that when there is in the apprehensive power a defect that is subject to the will, this defect also is deemed a sin. ”