Iago uses logos as a persuasive technique to convince Othello of Desdemona’s infidelity. By citing Desdemona’s past examples of deceit, Iago appeals to Othello’s sense of reason to show that she could also deceive him. He says to Othello:
She did deceive her father, marrying you,
And when she seemed to shake and fear your looks,
She loved them most
Here Iago uses the line of reasoning that Desdemona deceived her father; thus, she is intrinsically deceitful; therefore, she will also lie to Othello. This line of reasoning rests on a series of deductions that appeals to Othello’s desire for logic.
Iago also uses logos to convince Cassio to entreat Desdemona to help him make up with Othello after his fight with Roderigo in Act 2, Scene 3. Again he uses a series of deductions: Desdemona has great sway over her husband; Desdemona is of an "apt” and “free” disposition; Desdemona will therefore take kindly to Cassio’s suggestions of good will and mend things with her husband. By using supposedly reasonable discourse, Iago demonstrates his mastery of manipulation. By highlighting the logic of his suggestions, Iago convinces the other characters that they are acting in line with common reason, not with his influence, thus removing his own culpability in their actions.