When does Othello use reason or logic in the play?

Haha. I don't think that Othello uses much reason or logic. Instead, he decides based on his emotions. He tries to use reason, but ends up letting his jealousy and anger take over.

Without knowing it, Othello already believes Iago by the end of Act 3. He's demanded proof, but already the rumors and nagging at him. Soon, no amount of proof from Desdemona is going to fully convince him.

I guess that you could say he uses reason and logic in the beginning of the play when he comes across Brabantio and decides that fighting isn't the best solution.