Iago succeeds in making Othello terribly jealous, but the Moor is suspicious of Iago's motives. In Act 3, Scene 3, he threatens Iago with death. For example: "Villain, be sure thou prove my love a whore! / Be sure of it. Give me the ocular proof, / Or, by the worth of mine eternal soul, / Thou hadst been better have been born a dog / Than answer my waked wrath!" By a great coincidence, Iago gets possession of the precious handkerchief accidentally dropped by the innocent Desdemona and manages to pass it into Cassio's possession. Cassio gives it to Bianca, his mistress, and Iago, by various contrivances, manages to have Othello observe Cassio handling the handkerchief and joking about it and about Bianca. Othello mistakenly believes Cassio is joking about his lewd behavior with Desdemona. This is the only "oracular" proof Othello ever gets that Desdemona and Cassio are carrying on an affair. All this exaggerated valuation of a handkerchief and all the coincidences connected with it are very hard to swallow. They seem like a very contrived way of motivating Othello to murder the wife he adores. Somerset Maugham has the following to say about Othello's precious handkerchief in A Writer's Notebook:
"Shakespearian scholars would save themselves many a headache if when they come across something in the plays that is obviously unsatisfactory, instead of insisting against all reason that it is nothing of the kind, they admitted that here and there Shakespeare tripped. There is no reason that I can see to suppose that he was not well aware that the motivation in certain of the plays is so weak as to destroy the illusion....Why should he have put into Othello's mouth those lines beginning That handkerchief did an Egyptian to my mother give . . . unless it was because he was aware that the episode of the handkerchief was too thin to pass muster? I think it would save a lot of trouble to conclude that he tried to think of something better, and just couldn't."
Cassio's mistreatment of Bianca and his reputed philandering make us lose sympathy for him, and make us wonder what there is about him that makes Desdemona like him so much. His accidental possession of the handkerchief and his vulgar behavior with it are all far-fetched and only invented in order to give Iago a means of offering Othello the proof he demands in Act 3, Scene 3 in the most threatening terms.