Villain, be sure thou prove my love a whore;
Be sure of it. Give me the ocular proof,
Or, by the worth of mine eternal soul
Thou hadst been better have been born a dog
Than answer my waked wrath! (III.iii.)

In response to Iago’s insinuations about Desdemona’s adultery, Othello demands “ocular proof”—that is, evidence that will visually verify the allegation against his wife. Othello’s emphasis on the need for Iago to “prove [his] love a whore” demonstrates his commitment to justice. However, Othello’s commitment to law and reason also comes into direct conflict with his emotional and irrational response to Iago’s rumormongering. This conflict disrupts Othello’s otherwise black-and-white understanding of the world and leads to disastrous consequences.

It is the cause; it is the cause, my soul.
Let me not name it to you, you chaste stars;
It is the cause. (V.ii.)

Othello utters these words as he enters Desdemona’s chambers with the intent to kill. His refrain of “the cause, the cause” indicates his need to convince himself of the justness of his planned actions. However, this refrain also recalls Emilia’s words earlier in the play, where she claimed that jealous individuals “are not ever jealous for the [stated] cause.” In this light, Othello’s insistence on “the cause” may serve to conceal his innate jealousy and thereby subvert his commitment to legal procedure.

O perjured woman, thou dost stone my heart,
And mak’st me call what I intend to do
A murder, which I thought a sacrifice. (V.ii.)

Othello addresses these lines to Desdemona. His word choice here once again signals his commitment to the law. Calling Desdemona a “perjured woman,” he implies that by denying the allegations against her she has lied under oath. Desdemona’s supposed false testimony has dire consequences for Othello’s interpretation of her death. Whereas he sees her execution as lawfully necessary and therefore “a sacrifice,” if she is actually speaking the truth it would make him a murderer. The uncertainty she causes in him turns his heart to stone.