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Brutus, Cassius, and the philosophy of tyrannicide

On 15th March 44 BCE the would-be tyrant Gaius Julius Caesar was assassinated by 60 Roman Senators headed by Marcus Junius Brutus and Gaius Cassius Longinus. Brutus and Cassius are arguably one of the most famous pairs of heroes or rascals, depending on one’s point of view and on whether one accepts Shakespeare’s famous take on the events or not.
What interests me, and will provide the focus of this essay, are three questions: (i) what were Brutus and Cassius’ philosophical allegiances?; (ii) how does the assassination of Caesar square with such allegiances?; and (iii) what did the various Hellenistic philosophical schools have to say about the ethics of tyrannicide? My main source for this discussion is a gem of a technical paper published by David Sedley back in 1997 in The Journal of Roman Studies and entitled “The Ethics of Brutus and Cassius.” It is well worth a full reading.
Brutus is often assumed to be a Stoic, but he wasn’t (unlike his father-in-law, Cato the Younger, whom Seneca considered a role model). He was a Platonist, specifically an adherent of the so-called Old Academy founded by Antiochus of Ascalon in 80 BCE and attempting an update of Platonism that would include the Peripatetic school of Aristotle. Brutus studied under Antiochus’ brother, Aristus, and wrote a number of well regarded philosophical treatises, including De virtute (On virtue), De patientia (On patience), and On Proper Conduct (in Greek). In fact, Brutus was so admired as a philosopher in his own right that Cicero dedicated several of his books to him: De finibus, De natura deorum, Tusculan Disputations, and Paradoxa Stoicorum, as well as the dialogue Brutus.
Cassius, on his part, was an Epicurean, though he had embraced the philosophy only a few years before the assassination of Caesar, so was relatively new at it. We do not know what his philosophical allegiances were before then. This is interesting because the Epicureans were adamantly opposed to political involvement, on the ground that this causes pain, and a life without pain is the chief good for an Epicurean. Accordingly, Cassius had decided in 48 BCE to withdraw from the Republican (i.e., anti-Caesarian) struggle, but rejoined a few years later, evidently because he judged the circumstances to be sufficiently dire. His participation in the…