This question is regarding the play as a whole?
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I suppose Othello pretty much judges based on appearances mainly because he lets his immediate emotions get in the way. There is no sober second thought with Othello. Iago knows exactly how to subtly manipulate appearances so that Othello's mind reaches the conclusions that Iago wants. Othello thinks in absolutes. Desdemona is an untainted angel or a hoar, there is no in-between. This is why appearances have such a strong affect on him. The simple appearance or disappearance of a handkerchief, for example, is enough "proof" to set Othello off.