In the street that night, Cicero encounters Casca, who says he has seen many strange sights, including fire dropping from the sky and a lion in the Capitol, which he interprets as bad omens. Cicero asks if Caesar is coming to the Capitol the next day, and Casca says yes.
The supernatural events presaging Caesar's murder reflect its historical importance, and also raise the question of whether fate has decreed it inevitable.
Cicero exits and Cassius enters. Cassius says he's been walking in the storm unafraid, daring the lightning to strike him. Casca tells him he's unwise to tempt the Gods. Cassius says if Casca were a true, brave Roman, he'd understand that these omens are warnings about a certain man who, although he seems imposing, need not be feared because he's no mightier than they. Casca guesses he means Caesar.
The omens have definite meanings to the audience, because they know that Caesar will be murdered. The characters, in contrast, don't know what will happen, so they can interpret the omens to mean anything. Once again, Cassius manipulates conversation to make his own idea seem like someone else's.
Cassius says that the manly spirits of their Roman forefathers must be dead, with only those of women surviving, for things to have come to this. Casca says that the senators mean to make Caesar king the next day. Cassius says "I know where I will wear this dagger then: / Cassius from bondage will deliver Cassius" (1.3.88-9).
Ideas of right and wrong are closely tied to masculinity, as well as to tradition. Notice that Cassius is still only strongly hinting, rather than directly stating, that they should kill Caesar.
Casca agrees that those who are enslaved have the power to free themselves. Cassius says that Caesar could never have risen so high if other Romans were not so weak, and that Rome is "trash" if it will "illuminate / so vile a thing as Caesar" (1.3.107, 109-10). Casca says that he is willing to go as far as Cassius is.
Phrasing it in terms of slavery and weakness makes it seem like a moral duty to kill Caesar. Cassius continues gradually changing his description of Caesar from "immortal," to equal, to weak, to "vile."
Cassius says he's persuaded others to take up their cause, and that they wait for him at a theater erected by the defeated Pompey. Cinna enters and says the other conspirators are assembled. Cassius gives him letters to plant where Brutus will find them. Casca and Cassius discuss how Brutus is essential to their plan, because he's so respected that his name will lend legitimacy to whatever they do.
This scene highlights the difference between the other conspirators and Brutus: While they suspect that their plans are ignoble, and are complicit in Cassius's trickery, Brutus must be "fooled." Also, getting Brutus involved is essential to fooling the people, since everyone knows that Brutus is so morally upright.