This line opens the powerful Act V scene 2 of this excellent play, which comes as Othello enters Desdemona's bedchamber at night with a light, intent upon killing her. Let us examine the first three lines and see what light can be shed upon this opening speech:
It is the cause, it is the cause, my soul!
Let me not name it to you, you chaste stars,
It is the cause.
With the repetition that is used, refering to the "cause," or the reason behind Othello's desire to kill Desdemona, it is clear that Othello seems to feel the need to work himself into action, reminding himself, again and again, the reason behind what he is doing: the supposed adultery of his wife. In addition, the way that Othello seems to be addressing his "soul" in the first line suggests that he is talking to his deepest most intimate self. The fact that he seems to have to persuade this part of his psyche that he needs to kill his wife gives testament to the internal conflict that Othello is experiencing. The way in which Othello refuses to even mention the "cause" out loud, in case it sullies the purity of the "chaste stars," only serves to emphasise the rightness of "the cause."