CONSCIENCE, as a question belonging to psychology, was discussed by S. Thomas Aquinas in Pars Prima, lxxix. 12, 13. He found in man a habit of practical reason ("synderesis") by which we know the primary principles of things which are to be done; e.g., that good is to be followed and evil to be shunned. What that moral good and evil are is to be otherwise determined. But the act by which we apply to our own conduct our knowledge of good and evil, whether our judgment be correct or incorrect, is called conscience. In this act man testifies to himself respecting himself, holds himself bound or absolved, approves or condemns his own actions.
Divisions. Conscience is correct or erroneous; certain or doubtful; scrupulous or lax.
An erroneous conscience dictates falsely -- i.e., contrary to objective and binding law -- through ignorance of that law. (On ignorance in this regard, see Introduction, pages 12, 92, 466, seq.)
A correct conscience testifies, judges, approves, etc., in accordance with the objective law as it truly is. But it has only moral certainty in its judgments. Both a correct conscience and one invincibly ignorant, as we have seen, are to be followed, for it is always sin to act against one's conscience, whether it be correct or erroneous (Rom. xiv. 23 -- see Bishop Sanderson's excellent sermon on this text). But there may be sin in using its permission, since vincible error is wilful and sinful error. "The conscience hath power in obligations and necessities, but not so much nor so often in permissions" (Duct. Dubitant. I. i. rule 7; I. iii. rule 2).
But duty requires that one should earnestly and steadfastly seek for that outward light which illuminates conscience to see the right path for conduct (Whewell, El. Moral. iii. 365).
When should the priest enlighten an erring conscience? (See Supplement, page 599.) If the error be probably invincible, and the erroneous opinion which has been followed be consistent with a state of grace, the error must be opened or not according to prudent consideration of the person and his affairs. But let the priest beware of converting material sin into "formal" sin; and also of "casting pearls before swine" (Duct. Dubitant. I. iii. rule 8; I. iv. rule 14; op. Rom. vii. 7; 1 Tim. i. 13).
A doubtful conscience. Only a few words can here be added respecting that wide subject in Moral Theology known as "probabilism." Since opinion, with its uncertain judgments founded on fallible arguments, and with apprehension of a possible opposite, must often be our only guide to the right, and since in such a case we are compelled to act without moral (subjective) certainty that we are right in our choice of action, a few well-settled principles may be a useful clue in our difficult course.
(1) Where there is obligation of obtaining a determined end, it is not lawful to reject the surer and safer means of reaching it in order to follow what will probably enable us to reach that end.
This condemns, e.g., the popular Protestant sentiment, "We are all travelling to the same place, and it matters not what road we take." Again, the priest must apply this rule where questions arise respecting sacraments "generally necessary to salvation."
Again, this rule prohibits the physician's trying experiments on his patient if the healing art provide remedies which are morally certain to have good effect (Duct. Dubitant. I. iv. rule 3; I. v. rule 5).
(2) When the question is of the existence of an obligation or a law, a probable opinion may be followed, even if it be not the safer one, for an uncertain law does not bind conscience.
This moral principle, e.g., is calculated to meet a specious argument for perverting to the Roman communion -- sc., "You are not morally certain that it has no claim on you; but you admit that submission to the Roman see is the safer course." Unless you are morally certain that duty requires you to leave your religion and join that or any other, you sin in doing so. Therefore, you imperil your salvation by a course.
(3) Note here the difference between uncertainty of the law and uncertainty of the fact; for in the latter case the rule is precisely the opposite. In doubt whether you have filled an obligation under the law, that law may still be binding respecting that obligation.
(4) In doubt or uncertainty you may act on presumption if you were possessed of moral certainty. "Melior est conditio possidentis."
(5) If the principal and most essential fact be certain, but you are doubtful respecting other circumstances or necessary conditions, you may act upon probability; doubt is to be decided favorably.
(6) Decide, then, in favour of liberty if there be no contrary presumption, and the public good put no obstacle in the way of that liberty. But observe that civil law may be stricter than the moral law in this respect; e.g., in questions connected with the matrimonial contract.
(7) In doubtful cases, favours are to be regarded liberally, and extended as far as possible; penalties, on the other hand, are to be construed strictly; e.g., in case of an ecclesiastical trial.
Conscience may be perplexed, being compelled to choose between two evils. But observe the ambiguity in our proposition. If the "evils" be sins, you are not allowed to choose either of them; both must be rejected. But, on the other hand, the evils may be results of sins between which ("mala paenae") you are compelled to choose. Then applies the maxim, "Of two evils choose the least" (Sanderson's Praelect. ii. 18; Duct. Dubitant. I. iv. rule 3; I. v. rule 8).
The choice, e.g., may lie between continuing in a corrupted church and implicitly favouring apostasy and unbelief.
Again, a wife may have to choose between condoning a husband's infidelity to her by living with him, or breaking up a family.
Remember, also, that laws are of four different grades (page 503), and that the lower must always yield to the higher.
Ductor Dubitant. I. v. rule 8, defends the opinion that a lesser sin may be made a counsel to him who is bent upon a greater one. Thus, Pilate might rightly have counselled the Jews to scourge the Lord and let Him go; "not absolutely, but comparatively; that is, rather that than the other (the crucifixion), if ye will do one" of them.