Whether it is lawful to swear?

❌ Objection 1 : It would seem that it is not lawful to swear. Nothing forbidden in the Divine Law is lawful. Now swearing is forbidden (Mt. 5:34), "But I say to you not to swear at all"; and (James 5:12), "Above all things, my brethren, swear not." Therefore swearing is unlawful.

❌ Objection 2 : Further, whatever comes from an evil seems to be unlawful, because according to Mt. 7:18, "neither can an evil tree bring forth good fruit." Now swearing comes from an evil, for it is written (Mt. 5:37): "But let your speech be: Yea, yea: No, no. And that which is over and above these is of evil." Therefore swearing is apparently unlawful.

❌ Objection 3 : Further, to seek a sign of Divine Providence is to tempt God, and this is altogether unlawful, according to Dt. 6:16, "Thou shalt not tempt the Lord thy God." Now he that swears seems to seek a sign of Divine Providence, since he asks God to bear witness, and this must be by some evident effect. Therefore it seems that swearing is altogether unlawful.