A special case of foreseen effects is that in which one looks ahead and expects with greater or lesser probability that if one chooses to do X now, then later one is likely to do Y, when Y is something one ought not to do. If one nevertheless does X, one assumes some responsibility for doing Y, even if in the event one does not do Y. This mode of voluntariness was named “the voluntary in cause” by St. Thomas (see S.t., 1–2, q. 77, a. 7; 2–2, q. 43, aa. 1–3; q. 46, a. 2, ad 2).
There are at least two modes of voluntariness in cause. In one case, one foresees the likelihood that one’s choosing to do X now will lead to one’s choosing to do Y later. For example, a man might consider whether to stop at a stand where magazines containing immodest photographs are for sale, foreseeing that if he chooses to do so, he is likely to choose to buy one, look at it, and masturbate. In another case, one foresees the likelihood that one’s choosing to do X now will lead to one’s proceeding without choice to do Y later. For example, an alcoholic woman might consider whether to have a drink with an alcoholic friend, foreseeing that if she chooses to do so, they are likely without further deliberation to get drunk.
Voluntariness in cause also can apply to the foreseen consequences of one’s choices for the actions of another person or persons. For example, those who oppress others might foresee that their choice to do so is likely to lead to a choice by the oppressed to do violence. Again, one who chooses to tease an irascible person might foresee that the person is likely to react violently.
In principle, one’s responsibility for that which is voluntary in cause, whether in oneself or in others, is the same as one’s responsibility for other foreseen consequences of one’s acts. The difference is that, whereas the voluntary in cause bears upon goods of the existential domain, most foreseen consequences involve goods of other domains. In other words, in the present case, one is dealing with foreseen consequences which have in themselves an immediate moral significance, since they are cases of human action or of behavior which ordinarily can and ought to be avoided. The treatment of occasions of sin is concerned with voluntariness in cause in respect to one’s own foreseen actions and behavior; the treatment of scandal is concerned with voluntariness in cause in respect to the foreseen consequences of one’s acts for the actions and behavior of other persons.