In Othello, both formal and informal logical fallacies appear. Iago, the play’s central antagonist, uses numerous fallacies as he advances his plot to destroy the protagonist, Othello. Iago utilizes these devices in his vigorous campaign to manipulate many other characters, not just Othello. Two fallacies that Iago employs are the informal fallacy of appeal to emotion and the formal fallacy of unwarranted assumption.

Fallacy refers to faulty reasoning that renders an argument invalid or to faulty belief that is based on an unsound argument. Fallacies are likely to be deceptive, as they often seem to be based on sound reasoning and to use logic appropriately.

One place where the informal fallacy of appeal to emotion can be seen is in Iago’s initial efforts to manipulate Brabantio, Desdemona’s father, to oppose her marriage to Othello. This appeal includes underlying racism as well as anger as Iago urges Brabantio to search for the missing couple before his daughter gets pregnant.

Even now, now, very now, an old black ram
Is topping your white ewe. Arise, arise;
Awake the snorting citizens with the bell,
Or else the devil will make a grandsire of you.

A formal fallacy involving an unwarranted assumption is found in Iago’s goading of Othello to believe that Desdemona has been unfaithful. While it is true that she was seen with Cassio, Iago prompts him to leap to the conclusion that they are having an affair. He suggests that the idea was actually Othello’s all along.

I heard thee say but now, thou lik’dst not that.
When Cassio left my wife; what didst not like?
And when I told thee he was in my counsel.
In my whole course of wooing, thou criedst, “Indeed!”