On the Eighth Precept of the Decalogue

You shall not speak false testimony against your neighbor. Exod. 20. 16. - You shall not say false testimony. Matth. 18. 19.

437. • This precept, insofar as it is negative, prohibits, 1. chiefly and primarily, lest false testimony be said both in judgment and outside judgment; 2. indirectly and secondarily, lest the reputation and honor of the neighbor be injured.

But insofar as it is affirmative, it commands that, every simulation and fallacy being removed, simple and naked truth be said ¹.

¹ Thus the Roman Catechism (De VIII Praec., n. 2): "In this precept... two laws are to be noted, one prohibiting lest false testimony be said; the other commanding that, simulations and fallacies being removed, our words and deeds be measured by simple truth." But this part is drawn over to the affirmative precept so that that may be suggested which is common to all affirmative precepts, namely that they bind always, but not for always, since on the contrary it is often necessary to hide the truth.

Hence it prohibits, 1. perjury against the neighbor; 2. lying; 3. detraction; 4. contumely; 5. rash judgment and injurious suspicion. We must speak about each, except perjury, concerning which enough was said in the second precept.