Vawter, Marvin L. "'After Their Fashion':
Cicero and Brutus in Julius Caesar" Shakespeare Studies 9 (1976): 205 - 219.

Thesis: Cicero, who appears in Julius Caesar, wrote a work, De Divinatione, attacking the Stoic belief in divination. Vawter writes:
I am convinced that Shakespeare knew De Divinatione and used it to help shape the point of view in Caesar. Not only does Shakespeare have Cicero appear and speak lines only during the discussion of portents—and speak perfectly in character—but Cicero's catalogue of portents and practices of divination is much closer in detail and completeness to those in Caesar than in Plutarch's account. More important, Cicero's work offered Shakespeare extended commentary on and psychological insight into the fatalistic personalities of Roman Stoics, particularly the distorted, even perverse syllogisms with which stoics rationalized events—events they may only have imagined—as signs of Fate and "reasoned" about the future.  (206)
According to Vawter, Shakespeare, following Cicero, shows that the interpretations of portents reveals not what Fate has in store, but the personalities of those who do the interpreting. Thus Caesar, in choosing between Calpurnia's interpretation of his dream and Decius' interpretation, chooses the one most favorable to his image of himself. Furthermore, Vawter says that the supposed Stoic reasoning is mere rationalization. His prime example is Brutus' soliloquy on the reason that Caesar must die. Vawter writes that "actually the formal argument only appears to be a logical syllogism but is in reality a monstrous piece of rationalization, a tissue of words built to cover over subjective conclusion that Brutus has already drawn" (214).

Evaluation: It is not necessary to accept Vawter's assertion that Shakespeare must have read Cicero in order to see the validity of his arguments about Shakespeare's treatment of reasoning and the interpretation of portents. It seems to me indisputable that a great theme of the play is the ways in which people deceive themselves.

Bottom Line: Good stuff.