After Brutus leaves the stage, Cassius is left alone and makes a short speech or soliloquy (a speech representing his thoughts and not intended to be realistic speech). In the first sentence, Cassius states, “Thy honourable mettle may be wrought from that it is disposed” (303-04). The word mettle means quality or spirit, the word wrought means changed, and the word that is used for what. Cassius is thus saying that Brutus’ honorable nature can be easily changed from its natural state. The word mettle is also a pun for metal. This is an allusion to alchemy. In medieval times some scientists believed they could turn base metals, like lead or tin, into gold if they knew the secret. Here, Cassius is a reverse alchemist. He is attempting to turn Brutus’ gold (his honor) into tin. He is attempting to make Brutus dishonorable. Cassius explains that Brutus is too honorable, and this is his weakness. Honorable men should stay with only other honorable men (line 305), for dishonorable men like Cassius can easily corrupt them. Cassius adds that were the situation reverse, Brutus would never be able to influence him. An honorable man cannot change the dishonorable man.
Tin (dishonor) cannot be turned into gold (honor).
To ensure his success, Cassius decides to trick Brutus. Cassius plans to write several letters with different handwriting in each (lines 309-14). But each letter will basically state the same message: that the people of Rome are looking to Brutus to save them from tyranny. Cassius will then plant these letters in Brutus’ home so that Brutus will read them and then believe he has the support of many people in Rome to assassinate Caesar.