583. Since conscience that has the requisite conditions is our immediate guide in moral matters, it follows: (a) that a conscience which is true objectively, good, and certain must be followed, whenever it commands or forbids; (b) that a conscience which is in invincible error (see 30), but seems to him who has it to be not only true but certain, must also be followed when it forbids or commands. Examples: If a child were told and believed that he was obliged to tell a lie to prevent an evil, he would be bound to do this. If a person eats what he wrongly thinks to be forbidden food, he is guilty of the violation he apprehends.
584. Exception.—If invincible error results from lack of sufficient intelligence to be capable of sin (see above, 249, 387), then the failure to follow one’s conscience in such error does not make one guilty. Example: If a person unable to walk were persuaded that he was bound nevertheless to walk to church for Mass, his conscience would not make his omission sinful. Conscience supposes sane judgment, but the judgment we are now considering is not sane.
585. A conscience that has not the requisite conditions is not a safe guide, and hence it cannot be followed.
(a) An erroneous conscience may not be followed, if the error is vincible and there is danger of sin; neither may one act against it if there be danger of sin. To follow such a conscience would be to do what is wrong and to act in bad faith (i.e., to have a bad and erroneous conscience); not to follow it, would be to act against one’s judgment, wrongly formed though it was, and to do insincerely what is right (i.e., to have a bad, though true conscience). Example: A person who has made up his mind that dishonesty is necessary in his business, but who realizes that his reasons are not convincing, sins against sincerity if he follows his opinion; he sins against conviction, if he does not follow his opinion. But his predicament is due to his own sophistry or bad will, and the escape from it requires only that he be honest enough with himself to inquire about the matter.
(b) A doubtful conscience may not be followed, if the doubt is such that one is not reasonably sure that a certain act is lawful. Example: If a man does not know whether a certain remedy will be helpful or seriously harmful to another, his conscience is doubtful as to the lawfulness of administering the remedy, and it may not be followed. If in spite of this he makes use of the remedy, he is guilty of the harm he foresaw, even though it does not happen.
586. Exception.—It is lawful to follow a vincibly erroneous conscience, if there is no danger of sin in this. Example: If a person has neglected inquiry about holydays of obligation, and through his own neglect believes that Good Friday is a holyday, he does not sin by attending the services that day.
587. The signs of a vincibly erroneous conscience are: (a) that in the past one did not use the same diligence to inform oneself about one’s religious duties as is employed by conscientious persons; (b) that in the present one has fears, doubts or suspicions as to one’s own sincerity of judgment.
588. Results of Conscience.—The results of following an erroneous conscience are as follows:
(a) He who follows an erroneous conscience, commanding or forbidding or permitting, is not guilty of sin if his ignorance is invincible. Example: A child who thinks he is obliged to lie because he has been told to do this, is excused from sin on account of his ignorance.
(b) He who follows an erroneous conscience, commanding or permitting evil, is guilty if his ignorance is vincible. Example: A grown person who has persuaded himself that deception is lawful, obligatory or advisable, or that truthfulness is forbidden, but who ought to know better, is not excused by the conscience he has formed (see above, 97 sqq.).