Disclaimer: Caesar, Brutus and Cassius, as used here, probably belong slightly more to Shakespeare than to history, but certainly not to me. The Republic, Socrates and Adeimantos belong to Plato (for the curious, the sections referenced are 424 d-e and 401 d-e, respectively).
Caesar realized that he should have known better than to poke his nose into Plato again after all these years. The Academic had an alarming habit of writing things so absurd that one could not possibly believe them, but which somehow lodged themselves in a person's mind and worked their way around until one was obliged to give them at least some credit. It had been on account of Brutus that Caesar had revisited that subtle terror of his schoolboy days. To be sure, no long-dead, long-winded philosopher held any terror for Gaius Julius Caesar, conqueror of Gaul and Britannia, victor of a civil war, et cetera, but Plato's writings still produced that same unsettling effect that they always did. Nonetheless, he had looked into the Republic again because Brutus had brought up a philosophical question he wanted to discuss with Caesar and Caesar never denied Brutus anything. He was beginning to think, though, that there was one request he ought to have refused.
Looking over various portions of the dialogue, Caesar had stumbled quite unexpectedly upon a passage where Socrates and his companions were discussing music, apparently in conjunction with education. Someone other than Socrates had, for a wonder, managed to get a small paragraph in edgewise and his reflections, if taken seriously, were of a rather alarming nature. According to this Adeimantos, changes in the style of music caused changes in the characters of men who listened to that music. These alterations in people, in turn, impacted first their private dealings, then their public affairs, and ultimately the whole order of the state. The problem was apparently so serious that Socrates and Adeimantos were agreed that there must be rules to prevent any alterations in music, since changes there were the root of all greater troubles. Well, while Caesar had reformed everything from governance of the provinces to the Roman calendar, he was not going to try to regulate music - that was typical, impractical, Plato-type folly.
Even so, he could not help thinking of Caius Cassius, who simply did not listen to music, and wondering how he fit into Adeimantos' view of the matter. Certainly indifference to music was unusual, and since Cassius himself was in some ways strange, the two might well be connected. At social gatherings, Caesar reflected, Cassius almost always stood apart from the main crowds, isolated in sour observation. Even when Cassius did step forth to converse with friends - for there were men who claimed friendship with Cassius, astonishing though Caesar sometimes found this based on his own acquaintance with the man - he did so always with a disdainful expression, as if he separated himself from other men and their pleasures even in the act of associating with them.
What sort of man must he be, who did not delight in music and the common entertainments of his fellows? Music, after all, was supposed to be the most penetrating force, able to reach more deeply into a man's soul, and have a stronger hold there than anything else . . . . Caesar had a recollection of some two or three phrases to that effect in Greek - quite probably, he thought, in Socrates' typical didactic style - which could even have come from the Republic, though he was unable to locate the passage with a cursory search and was disinclined to try further.
Setting aside the effect - or non-effect - of music on Cassius in his personal affairs, there still remained the problem of Cassius as a Senator. True, he might not shift his political opinions because of anything so commonplace as listening to a new melody, but perhaps there was, equally, no way to keep such a man in check, so much given to thinking as he was, and so closed to other men's thoughts. It might have been wise to have denied Brutus' request, in the wake of Pompey's defeat, that Cassius be pardoned for his part on Pompey's side.