“Excellent wretch: Perdition catch my soul But I do love thee: and when I love thee not, Chaos is come again.”
Othello seems far more comfortable expressing his love for Desdemona when she is absent. Perhaps this is because her presence makes him conscious of her claim upon him and of his obligation to honour her requests, or perhaps this is because he is more in love with some idea or image of Desdemona than he is with Desdemona herself. The lines just quoted indicate how much his image of her means to him: if he stops loving her, the entire universe stops making sense for him, and the world is reduced to "Chaos."
Given how much is at stake for Othello in his idea of Desdemona, it is remarkable how he becomes completely consumed by jealousy in such a short time. Moreover, it takes very little evidence to convince him of her unfaithfulness. All Iago has to do to Othello is make him doubt Desdemona, and jealousy spreads like a virus until he rejects her absolutely. Notably, Iago, too, has no evidence that Othello has slept with Emilia, but the suspicion or doubt seems to have been sufficient to make him spurn Emilia and persecute Othello. As Othello says,
"To be once in doubt, Is once to be resolv’d”
Othello soon learns, however, that to be once in doubt is to be never resolved. He leaves the stage briefly after the episode in which he rejects Desdemona's handkerchief, at which point he seems resolved that his wife no longer loves him. A mere forty lines later, he returns, and all he can think about is garnering proof of her infidelity. The paradox in Othello's situation is that there are few things—the nature of friends, enemies, and wives included—that a human being could know with certainty. Most relationships must be accepted based on faith or trust, a quality that Othello is unwilling to extend to his own wife. All Iago really has to do to provoke Othello is to remind him that he doesn't know for certain what his wife is doing or feeling. Iago's advice that Othello
"Look to his wife. Observe her well…”
appears harmless at first, until one considers how out of the ordinary it is for a husband to "observe" his wife as if she were a specimen under a microscope. For a man to treat his wife as a problem to be solved or a thing to be known, rather than as a person with a claim upon him is simply incompatible with the day-to-day business of being married. Othello's rejection of his wife's offering of physical solace (via the handkerchief), and his termination of the exchange in which Desdemona argues for Cassio, thereby asserting a marital right, clearly demonstrate this incompatibility.
Ironically, Iago doesn't have to prove his own fidelity to Othello for Othello to take everything Iago suggests on faith. On the contrary, Othello actually infers that Iago holds back more damning knowledge of Desdemona's offences out of his great love for Othello. Again and again, Iago insists that he speaks out only because of this love. His claim,
"My lord, you know I love you"
even echoes Peter's insistent words to Christ,
"Lord, thou knowest that I love thee" (John 21:15–17)
Othello's rejection of Desdemona's offer of her handkerchief is an emphatic rejection of Desdemona herself. He tells her he has a pain "upon" his forehead and dismisses her handkerchief as "too little" to bind his head with, implying that invisible horns are growing out of his head. Horns are the traditional symbol of the cuckold, a husband whose wife is unfaithful to him. Othello's indirect allusion to these horns suggests that the thought of being a cuckold causes him pain but that he is not willing to confront his wife directly with his suspicions.
The end of Act III, scene iii, is the climax of Othello. Convinced of his wife's corruption, Othello makes a sacred oath never to change his mind about her or to soften his feelings toward her until he enacts a violent revenge. At this point, Othello is fixed in his course, and the disastrous ending of the play is unavoidable. Othello engages Iago in a perverse marriage ceremony, in which each kneels and solemnly pledges to the other to take vengeance on Desdemona and Cassio. Just as the play replaces the security of peace with the anxiety of domestic strife, Othello replaces the security of his marriage with the hateful paranoia of an alliance with Iago. Iago's final words in this scene chillingly mock the language of love and marriage:
"I am your own forever"