Augustus made a law that the slaves of those who conspired against his person should be sold to the public, that they might depose against their master. 44 Nothing ought to be neglected which may contribute to the discovery of a heinous crime; it is natural, therefore, that in a government where there are slaves they should be allowed to inform; but they ought not to be admitted as witnesses.
Vindex discovered the conspiracy that had been formed in favour of Tarquin; but he was not admitted a witness against the children of Brutus. It was right to give liberty to a person who had rendered so great a service to his country; but it was not given him with a view of enabling him to render this service.
Hence the Emperor Tacitus ordained that slaves should not be admitted as witnesses against their masters, even in the case of high treason: 45 a law which was not inserted in Justinian's compilation.