The means used to obtain an end are very important in a consideration of the morality of an act. There are four classes of means--the good, bad, indifferent, and excusable. Good means may be absolutely good, but commonly they are liable to become vitiated by circumstances,--almsgiving is an example. Some means are bad always and inexcusable--lying, for instance. The excusable means are those which are bad, but justifiable through circumstances. To save a man's life by cutting off his leg is an excusable means. The end sometimes may vitiate or hallow indifferent means, but it does not in itself justify all means. Means, like other circumstances, are accidents of an action, but they are in the action just as much as color is in a man. Color is not of a man's essence, but we cannot have a man without color.
The effect of an action, the result or product of an effective cause or agency, may in itself be an end or an object or a circumstance, and it has influence in the determination of morality. Sometimes an act has two immediate effects, one good and the other bad. For example, ligating the blood-vessels going to the uterus to stop a hemorrhage and so save a woman's life, a good effect, has also in ectopic gestation while the fetus is living another immediate effect, namely, to shut off the blood supply from the fetus and so kill it, a bad effect. To make such a double-effect action licit there are four conditions which are explained in the chapter on Mutilation.
The doctrine of Probabilism is very important in morality. Any law must be promulgated before it really becomes a law, and promulgation in a rational conscience is sufficient. Sometimes there is rational doubt of the existence, the interpretation, or the application of a law in a given case. Here probability is the only rule we can follow. A law which is doubtful after honest and capable investigation has not been sufficiently promulgated, and therefore it cannot impose a certain obligation because it lacks an essential element of a law. When we have used such moral diligence as the gravity of the 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 where 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 doubtfully efficacious, he may not choose this probable cure. Probabilism has to do only with the existence, interpretation, or applicability of a law, not with the differentiation of actions. The term probable means provable, not guessed at, not jumped at without reason. The doubt must be positive, founded on reason, not a matter of mere ignorance, suspicion, emotional bias. The opinion against a law to permit probabilism must be solid. It must rest upon an intrinsic reason from the nature of the case, or an extrinsic reason from authority, always supposing the authority is really an authority. The probability is to be comparative also. What seems to be a very good reason when standing alone may be weak when compared with reasons on the other side. When we have weighed the arguments on both sides, and we still have a good reason for holding our opinion in a doubtful case, our opinion is probable. The probability is, moreover, to be practical. It must have considered all the circumstances of the case.