Cassius’s words to Brutus in Act I, scene ii have proved powerful in turning him against Caesar: while alone in his garden, Brutus has come to the conclusion that Caesar must be killed. The forged letter has secured this conversion; though it has appeared so mysteriously in his house and tells him exactly what he wants to hear, Brutus never questions its authenticity. He immediately construes the message’s cryptic meaning according to his preconceived inclinations: “Thus must I piece it out,” he concludes hastily, allowing for no other interpretation of the words (II.i.51). He displays a tragic naïveté, trusting unquestioningly that the letter speaks for the entire Roman populace.
We see now that once Brutus arrives at a belief or proposition, he throws himself into it wholeheartedly. Upon joining Cassius’s conspiracy, he takes control of it. He provides his own garden as the conspirators’ meeting place and convinces the gathered men not to take an oath, though Cassius would prefer that they do so. Brutus is the one who sends Decius to speak to Caesar at the end of the scene, and it is he who speaks the final words to the conspirators as they depart. So, too, does Brutus overrule Cassius when he suggests that they assassinate Antony along with Caesar. This position, like all of Brutus’s actions, stems from a concern for public opinion: Brutus wants the death of Caesar to appear an honorable gesture; if the scheme became too violent, the conspirators would sacrifice any semblance of honor. He insists rather excessively on preserving honor in the conspiracy, saying that in a noble cause one has no need to swear an oath to others: “Do not stain / The even virtue of our enterprise, / Nor th’insuppressive mettle of our spirits, / To think that or our cause or our performance / Did need an oath” (II.i.131–135). Men swear oaths only when they doubt the strength of each other’s devotion; to take up oaths now would be to insult the current undertaking and the men involved. It is a rather ironic proposition from Brutus, who has declared loyalty and friendship to Caesar and now casts those commitments aside. Notably, Brutus asks the men not to “stain” the virtue of their scheme, a word that evokes blood; ultimately, they will not be able to avoid staining themselves with Caesar’s blood.
Yet, although Brutus appears completely determined in his interactions with the conspirators, his inability to confess his thoughts to Portia signifies that he still harbors traces of doubt regarding the legitimacy of his plan. Portia is a symbol of Brutus’s private life—a representative of correct intuition and morality—just as Calpurnia is for Caesar in the next scene. Her husband’s dismissal of her intuitions, like Caesar’s of Calpurnia’s, leads to folly and points to his largest mistake: his decision to ignore his private feelings, loyalties, and misgivings for the sake of a plan that he believes to be for the public good.