Summa summae S. Thomae, Tomus II

[It must be noted that this question has undeservedly been held by many theologians to pertain pertinently to probabilism, whose error arose from this, that there intervened here a certain probability to whose object they paid insufficient attention.

Certain probabilists, after they had demonstrated their system with valid reasons, lest they seem to transgress the limits of just moderation, established certain exceptions, as they said, to which the principle of probabilism may not licitly be applied, v. g., the case of the sacraments validly to be conferred out of necessity, the obligation incumbent upon a physician to use the more efficacious means for restoring the health of the sick person, etc. They judged that, with these exceptions admitted, their theory would become unshaken, not considering that, if from their doctrine there flowed false consequences which they rejected in practice, the principle itself would be convicted of falsity.

And in fact the antiprobabilists, leaning on these exceptions, tried to overthrow the system by way of absurdity, reasoning thus : A principle that leads to false consequences is false. But the principle of probabilism leads to consequences so evidently false that they are rejected by the probabilists themselves. Therefore this principle is false. And to prove the minor they alleged the cases recalled above. Indeed this conclusion would have to be acknowledged as well founded, if the various cases comprehended under the present question were only exceptions in the aforesaid sense : which is false.

That this confusion may be dissipated, there must be recalled to memory the distinction which we have already had occasion several times to note. In the system of probabilism, the question concerns exclusively the liceity of an act posited against a law doubtful or only probably existing, in which case the obligation, which is the essential effect of the law and is necessarily proportioned to and homogeneous with the law, is itself probable or doubtful. But if it be truly and solidly probable that the law does not exist, from the reflex principles whose truth was demonstrated above it is rightly pronounced that the obligation is none, because the law is practically none, so that the agent who omits such a law does not sin formally. — On the contrary, in the question now to be solved, the law commanding that some determinate end be obtained, and indeed by efficacious means, is not doubtful, but altogether certain, and from it consequently arises an equally certain obligation. It is supposed that there are simultaneously present means certainly conducing to the end, and other means by which the same end can be attained only probably, and the question is whether it is lawful, leaving the former, to employ the latter : i. e., whether, the certain obligation still remaining, the act itself avails.

That this discrepancy may be expressed more briefly : the probabilist treats of the liceity of an action posited against a doubtful law, and in the present question there is treated of the validity of an act posited by force of a certain law. And these two differ as heaven from earth. Therefore I say : It is not lawful to follow a probable, or even a more probable, opinion, leaving the safer, when there is present a certain obligation of obtaining a determinate end.

Proof. When there is present a certain obligation of obtaining a determinate end, the agent is bound to employ those means by which he will attain this end in the most efficacious way possible. But if means certainly efficacious are at hand, and the agent chooses means whose efficacy is only probable, he does not attain the end in the most efficacious way possible. Therefore he does not fulfill the obligation imposed upon him. Therefore it is not lawful to use these means, but the safer part must be followed.

Thence you will gather :

1° In matters necessitated by the necessity of means for salvation, the safer must be chosen. For the gravest and most certain obligation is imposed upon everyone to make certain the eternal salvation of his soul. But means only probably efficacious do not make the salvation of the soul certain. Therefore, leaving the others, it is not lawful to employ these means. Hence in matters pertaining to faith and the true religion, all probability being set aside, the safer part must necessarily be followed. Whence this proposition was condemned by Innocent XI, n. 4 : « An infidel not believing will be excused from infidelity, led by a probable opinion. » The same must be said of an opinion even more probable subjectively. So St. Liguori, lib. I, n. 43.

2° The minister of a sacrament is bound to make its validity certain, embracing the safer part, leaving the less safe opinion, though probable and even more probable. The reason is, because the honor and reverence of the sacrament are entrusted to him, and together with them the spiritual good of the neighbor, who has the right not to be deprived of the grace which he needs or desires to enjoy. But by acting otherwise, the minister would expose the sacrament to the danger of nullity and the neighbor to the proximate danger of spiritual loss. Whence the following proposition was deservedly condemned by Innocent XI, n. 1 : « It is not illicit, in conferring the sacraments, to follow a probable opinion about the value of the sacrament, leaving the safer, unless the law, a convention, or the danger of incurring grave loss forbids it. Hence an opinion merely probable is not to be used in the conferring of baptism, of priestly or episcopal order. » — Excepted is 1° the case of urgent necessity, because, as the received axiom runs, the sacraments are for men, and to a man placed in grave danger of death they must be administered in the best way possible, even with mere probability, nay with a thin and very thin probability regarding validity. Whence baptism can not only be, but must be, conferred on one about to die shortly with liquid doubt, if natural water is lacking and cannot be obtained. — Excepted is 2° the case in which, on account of necessity, the defect is presumed to be supplied by the church; for sometimes the Church can and is held to supply what is wanting to the value of the sacrament. Which indeed can happen in two sacraments, namely in the sacrament of penance, if for a grave reason it be conferred with doubtful or only probable jurisdiction, and in the sacrament of matrimony, when it is celebrated with a doubted impediment of ecclesiastical law. S. Lig., n. 48-50.

3° He who has undertaken toward another an obligation from an express or tacit contract, or from a quasi contract, is bound to fulfill it in the more certain and safer manner. Whence the physician and the surgeon must employ the more efficacious remedies and means that are available here and now, because for this they are summoned by the sick. S. Lig., n. 44.

4° When perilous action is directed against someone, one must abstain from it, unless true necessity excuses it, or the public good requires it. Whence a hunter ought not kill a wild beast, if he fears lest by the same blow he kill or merely wound a man. And he must abstain from it, even if he judge it more probable that the danger is absent; for if in truth it be present, it will not be removed by a contrary probability, even though greater.]