Much might be said here concerning conscience as a judge of the morality in an act, but this discussion is not necessary for our present purpose. Like other men, physicians often confuse conscience with inclination, or at best with unfounded opinion. When conscience is to be a rule of action it must {16} have at the least moral certitude; or, what is different but practically the same thing, the opinion of conscience must be at the least genuinely probable. The term "probable" is used here in a technical sense, and it will be so used throughout the remainder of this article. The doctrine of Probabilism is connected with the promulgation of law. A law, according to St. Thomas (_op. cit._ I. 2., q. 90, a. 4) is: "Ordinatio rationis ad bonum commune ab eo qui curam habet communitatis promulgata." Sometimes it is not evident whether or not a law binds in a particular case, and in such a condition, that is, in which there is question solely of the existence, interpretation, or application of a law, we may follow a probable opinion which assures us the act is licit, although the opinion which says the act is illicit may be just as probable or even more probable. This is the fundamental proposition of Probabilism, which is the doctrine especially of St. Alphonsus Liguori, but it was held centuries before his time. As the church has never condemned this doctrine, but rather tacitly approved of it, Catholics may safely follow it, and those that are not Catholics will find it very reasonable. A law which is doubtful after honest and capable investigation has not been sufficiently promulgated, and therefore it can not impose a certain obligation because it lacks an essential element of a law. When we have used such moral diligence of inquiry as the gravity of a matter calls for, but still the applicability of the law is doubtful in the action in view, the law does not bind; and what a law does not forbid it leaves open. Probabilism is not permissible when there is question of the worth of an action as compared with another, or of issues like the physical consequences of an act. If a physician knows a remedy for a disease that is certainly efficacious and another that is probably efficacious, he may not choose the probable cure, at the least in a grave illness. Probabilism has to do with the existence, interpretation, or applicability of a law, as I said, not with the differentiation of actions.