Tacitus: Agricola 2: 1-2
We have read that when Paetus Thrasea was praised by Arulenus Rusticus and Priscus Helvidius by Herennius Senecio, it was a capital offence and not only the authors themselves but even their books were treated savagely. The task having been assigned to the board of three, these monuments to the great fame of men of genius were burnt in the comitium and the forum. One might have thought that in that fire the voice of the Roman people and the liberty of the Senate and the moral consciousness of the human race had been wiped out. What is more, philosophers were expelled and every noble practice was sent into exile so that no virtuous acts could occur.
Paetus Thrasea was condemned to death under Nero.
Arulenus Rusticus (aka Junius) wrote a panegyric on Paetus Thrasea, for which he was condemned to death by Domitian.
Priscus Helvidius was condemned to death by Vespasian. His biography was written by Herennius Senecio, who was also condemned to death under Domitian.
In the same year as Arulenus Rusticus and Herennius Senecio were condemned to death Domitian expelled philosophers (i.e., intellectually-minded dissidents) from Rome. It is possible that Plutarch was one of those expelled.
This crushing of political and ideological dissent from Domitian's reign left deep scars, and was perhaps one of the reasons that led to the composition of Tacitus' biography of Agricola, to justify the fact that Agricola and Tacitus had continued to serve the regime despite its oppressive nature.
