1. The primary case of a sin of thought is a sinful choice.12 Even if it is a choice to say or do (or omit) something, the sin is already present in the choice itself, and one who is prevented from executing the choice or repents before doing so has nevertheless committed a sin of thought (see S.t., 1–2. q. 74, a. 7). (Other things being equal, of course, a sinful choice which is not carried out is less evil than one which is.)
2. Sometimes one makes a sinful choice subject to a contingent condition not in one’s own power: I will do X if I have the chance, if it does not seem too risky, if somebody else does Y, and so on. The sin of thought is committed whether or not the condition is fulfilled and the choice carried out.
3. The wish to do or have something evil is itself evil. The desire to commit a mortal sin one cannot commit or to have committed a mortal sin one can no longer commit is grave matter. Persistence in such desires and wishes is a mortal sin if one reflects sufficiently and chooses to entertain them.
4. If, however, one experiences desires contrary to a firm choice to set them aside, such experiences are emotions and not wrong choices. Moreover, to know as a matter of fact that one would like to commit certain sins or that one feels sad at having forgone the pleasure of sinning is not to desire deliberately to commit sin. A person who thinks “I would do X if it were not a sin” ordinarily does not will to commit the sin but is unwilling to do so.
5. It is evil to consider with satisfaction and approval the doing of something evil, whether by oneself or another. Hence, such consideration of the actual or possible doing of a mortal sin is itself grave matter. One aware of taking satisfaction in or approving sin ought to choose to stop doing so. If, having sufficiently reflected, one instead chooses not to set aside this sinful attitude, the continuing satisfaction or approval is mortally sinful (see S.t., 1–2, q. 74, a. 8).
6. It is possible, however, to enjoy knowing about evil without taking satisfaction in it. One may, for instance, enjoy a story about a robbery and be delighted with the robber’s skill, without taking satisfaction in the sin or approving it.
7. One can also take satisfaction in and approve the residual good aspects of an evil act, without approving of the evil. So one might be pleased by the acquittal of a woman who murders her brutal and unfaithful husband and happy about the deterrent effect which the act will have on other men of this sort, without approving homicide.
8. One can also consider with satisfaction and approval an act which it would be wrong to carry out now but which will be or was good at the time to which one refers it. Thus, an engaged couple can blamelessly look forward to marital intercourse while a widow can blamelessly look back on her intimacy with her husband.
It does not follow that people can rightly choose to take emotional satisfaction in thinking about doing things under conditions in which they would be right, when these things may not now be done without sin, and when there is no real necessity to think about them. One usually should judge that to engage in such thinking is wrong, because it will lead to temptations to do something wrong; to choose to engage in it despite such a judgment is a sin.
Satisfaction in the knowledge or in good aspects of sinful acts one would not oneself be tempted to choose can be distinguished readily enough from satisfaction in or approval of the evil as such. Thus, most people can read stories of robbery written from the point of view of the criminal without adopting a frame of mind which is wrong.
If, however, one considers immoral acts one would oneself be tempted to choose, supposing they seemed to be real possibilities, then satisfaction in the knowledge or in good aspects of the subject matter cannot easily be distinguished from satisfaction in or approval of the evil as such. One who thinks a state of mind could be wrong yet chooses to enter or persist in it is willing to do what is wrong. Thus, most people cannot read stories of illicit sexual activities without adopting a frame of mind which is wrong.
12. In some respects, the treatment of sins of thought here depends upon the common views of the classical moral theologians. See Vermeersch, op. cit., sections 429–35; Aertnys and Damen, op. cit., sections 241–47; Merkelbach, op. cit., sections 452–60.