Other forces in the world of this play help to make Othello choose as he does. The workings of chance help Iago—for example, Desdemona’s accidental loss of her handkerchief. Human fallibility helps Iago—for example, Emilia’s desire to gain a kind word from him by giving him the lost handkerchief. People’s own virtues may undo them—Iago uses Desdemona’s generous sympathy for Cassio against her. The fertility of other people’s imaginations aids Iago—their ability to make their own monsters. But Iago also makes use of Othello’s demand for “ocular proof” that Desdemona is untrue. Without actually offering any material evidence, Iago makes Othello “see” what he fears by spinning a story of Cassio talking in his sleep about a tryst with Desdemona. The “ocular proof” that Othello is eventually given is an interchange between Cassio and Iago that Othello does not fully hear. Iago’s fabrication of this proof is helped by the fortuitous appearance of Cassio’s mistress, Bianca, brandishing Desdemona’s handkerchief. Othello sees nothing of Desdemona’s supposed infidelity, only the lost handkerchief. The object becomes equated with her lost honor and with Cassio’s supposed possession of her. Othello’s choice to believe that Iago is honest and Desdemona false has immediate consequences. He invokes the infernal powers of vengeance and enters into a terrible covenant of revenge with Iago. He transforms Desdemona into a “fair devil” whose white skin masks blackness within. He asserts that the purity of his own name has been begrimed by Desdemona’s actions.