This question is regarding the play as a whole.
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Othello seems to be the one who has the most difficulty distinguishing appearance from reality. His gullibility is easily "played" by Iago who makes Othello believe things that Othello sees which have Iago's meaning rather than the meaning of what is true. Iago is able to get Othello to see emotions that do not exist, affairs that are not happening, and lies when there is only truth. He does not see through the lies of Iago, when, in fact Iago is the biggest liar. Othello does not believe the truthfulness of Desdemona who is the one he should believe.
Appearance vs. reality
Especially relevant to the issue of Iago's character; for although he is called "honest" by almost everyone in the play, he is treacherous, deceitful, and manipulative. Also applies to Desdemona, as Othello believes that she is deceitful and impure, although she is really blameless and innocent.