Montesquieu

It was esta­bli­shed by Augustus that the sla­ves of those thought to have cons­pi­red against him should be sold to the public so that they could depose against their mas­ter.1 Nothing must be neglec­ted that leads to the dis­co­very of a great crime ; thus, in a state where there are sla­ves, it is natu­ral that they should be pos­si­ble infor­mers. But there is no way they can be wit­nes­ses.

Vindex infor­med on the cons­pi­racy for­med in favor of Tarquin, but he was not a wit­ness against the chil­dren of Brutus. Giving his free­dom to a man who had ren­de­red such a great ser­vice to his coun­try was just, but it was not given to him in order for him to ren­der this ser­vice to his coun­try.

And so did the empe­ror Tacitus decree that sla­ves should not be wit­nes­ses against their mas­ter even in the crime of lese-majesty,2 a law which was not inclu­ded in the Justinian com­pi­la­tion.

Dio, in Xiphilinus.

Flavius Vopiscus, in his life.