Explain why Othello believed he did not need to find his own proof of Desdemona's betrayal because of his complete trust for Iago. Include specific quotations and examples.

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Othello trusted Iago. He had more faith in Iago than he did his own wife Desdemona. The question is why did Othello trust Iago more than  his own wife? Clearly, Iago is a very persuasive man. He begins filling Othello's head with doubts about Desdemona's faithfulness in marriage. Iago uses his intellect to further his plot against Othello:

Othello's confidence starts to slip when Iago begins to work on his psyche, intimating that Desdemona and Cassio are having an affair.

Iago begins planting seeds of jealousy in Othello by pointing out that Desdemona married Othello while deceiving her own father. Iago points out that Desdemona is a deceitful person. Then Iago begins tearing down Othello's self esteem:

 After Iago has suggested that Desdemona has already deceived her father and Othello, the Moor begins to think Desdemona's betrayal of him is inevitable given his skin color, greater age, and lack of courtly charm (III.iii.263-268). He begins to act as if her unfaithfulness is a certainty, bemoaning that "Othello's occupation is gone" (III.iii.357).

Iago works Othello into a jealous rage through mind games. Iago is clever. He uses damning physical evidence to support his scheme. Iago plants Desdemona's handkerchief at Cassio's home. This act leads to Othello's rage:

Othello focuses on this piece of cloth as damning physical evidence in his confrontation with his wife. He refers to it repeatedly before he kills Desdemona: "That handkerchief which I so loved and gave thee, / Thou gav'st to Cassio" (V.ii.48-49); "By heaven, I saw my handkerchief in 's hand" (V.ii.62); and again, "I saw the handkerchief" (V.ii.66).

Iago is manipulative. He knows Othello's weak points. He plays on Othello's weaknesses. In the end, Othello is convinced that his wife has played the part of a harlot:  

Othello is so convinced by Iago's manipulation that he murders his wife in their bed. The most apparent reason for this deed is the one Othello gives to Emilia, stated repeatedly in response to her persistent questioning, immediately after he has smothered Desdemona: "She turn'd to folly, and she was a whore"; "She was false as water"; "Cassio did top her" (V.ii.133; 135; 137).

Truly, Othello trusts Iago. He has complete faith in him as his ensign:

I assign my wife
To the care of my ensign,
A man of honesty and trust,

No doubt, Othello puts his complete confidence in his ensign Iago. Othello does not want to believe Iago. In fact, he determines he must have proof. That is when Iago uses Desdemona's handkerchief as damning, physical evidence. Iago is brilliant in his scheme. He convinces Othello that Desdemona and Cassio have been having an affair. 

Othello thinks he knows Iago. At the same time, Iago truly knows his long-time superior and exactly how to manipulate him.