We have several reasons for assuming that Brutus is sincere when he shows reluctance to participate in killing Caesar and that he is sincere in saying that he knows of no personal cause to spurn at him.
- Brutus is a sincere man. He has a reputation for integrity, and we feel his sincerity in everything he says. We do not question his sincerity in his private thoughts. After all, he is talking to himself. Why should he be trying to deceive himself?
- Brutus demonstrates his reluctance to get involved in the assassination by taking such a long time to think about it. He is agonizing over the question of what to do. It is because he has "no personal cause" that it is so hard for him to decide. It is because he has "no personal cause" that the conspirators want to recruit him. They are thinking about how the Roman people will react to Caesar's killing. Brutus can lend legitimacy to the assassination and also lend legitimacy to the new government they intend to establish.
- Brutus and Caesar are good friends. Brutus could not be harmed if Caesar acquired more power but could only benefit. Cassius, who is never concerned about anyone but himself, observes: "Caesar doth bear me hard, but he loves Brutus. / If I were Brutus now, and he were Cassius, / He should not humor me." Cassius is saying, in effect, that Brutus has "no personal cause" to act against Caesar. This is why Cassius, practically in the same breath, says that he will deliver forged letters to Brutus' home "...all tending to the great opinion / That Rome holds of his name..." Cassius hopes to appeal to Brutus' patriotism and family pride in order to sway him in favor of the conspiracy.
- Antony, who was Brutus' enemy, speaks highly of him at the end.
This was the noblest Roman of them all.
All the conspirators, save only he,
Did that they did in envy of great Caesar;
He only, in a general honest thought
And common good to all, made one of them.
His life was gentle, and the elements
So mix'd in him that Nature might stand up
And say to all the world, “This was a man!”
Act 5, Scene 5