640. As was said above, only that conscience is a safe guide which is not only good--that is, in agreement, as far as one’s efforts can secure this, with the external law--but also certain. A certain conscience is one which, without any prudent fear of erring, judges that a particular act is obligatory or unlawful, and hence here and now to be done or omitted. 641. Necessity of Certitude.--We must be sure we are right before we act; otherwise, we expose ourselves to the danger of sinning, and therefore commit sin (see 582). Hence, it is necessary to act with a certain conscience, and unlawful to act with an uncertain conscience. “If the trumpet give an uncertain sound, who shall prepare himself to the battle?” (I Cor., xiv. 8) may be accommodated to conscience. In Rom., xiv. 22, 23, the Apostle declares that he who acts with conviction is blessed, whereas he who acts in uncertainty is condemned. Examples: Sempronia doubts whether it is sinful to sew on Sunday; she is not sure, but has grave suspicions that sewing is servile work; if she goes ahead, she will be guilty of violating the law, as being willing to take the risk, and therefore the responsibility. Titus offers another a drink, being uncertain whether it has poison in it or not; he is guilty of sin, since he has no right to expose himself to sin and his neighbor to the danger of death. 642. Those persons who act with a doubtful conscience, and later discover that what they feared might be wrong was not wrong, or not so bad as they suspected, must bear in mind: (a) that their past conduct is not to be judged by their newly acquired knowledge, for that conduct must be judged by the knowledge had at the time. Example: Sempronia does some work on Sunday, doubting whether she is committing a grave or a slight sin. Later she discovers that it was really only a venial sin, and she congratulates herself that she did not sin seriously. Her judgment is wrong, because she did not know at the time of the work that it was not a grave sin; (b) that they must guide themselves in future acts by their newly acquired knowledge. 643. Kinds of Certitude.--Judgments may be certain in a greater or less degree. (a) They are metaphysically certain, when error is absolutely impossible, the opposite of what is held by the mind being a contradiction in terms which omnipotence itself could not make true. Example: The judgments that the same, identical act cannot be both good and bad, that good is to be done and evil to be avoided, that God is to be honored, are metaphysically certain, since they result immediately from the very concepts of being, of goodness, and of God.