Act 3 scene 4
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I think you are referring to this quote,
But jealous souls will not be answer'd so;
They are not ever jealous for the cause,
But jealous for they are jealous: 'tis a monster
Begot upon itself, born on itself.
Emilia says that alpha-men like Othello don't need a reason to be jealous. Othello's jealousy and paranoia feeds on itself. Othello only needs a seed of doubt to germinate in his head before it becomes a full blown obsession.