“I have already chose my officer.” And what was he? Forsooth, a great arithmetician, One Michael Cassio, a Florentine, A fellow almost damn’d in a fair wife, That never set a squadron in the field, Nor the division of a battle knows
This sentence shows a deep theme for Othello and for Lago's character as a whole, the idea of jealousy. The way Lago describes Cassio is that of someone lesser than himself and maybe even lesser a person. Attacking his origin, a "Florentine" as some form of insult, calling Cassio not one of us. Also he attacks Cassios Battle knowledge saying he's "Never set a squadron in the field Nor the division of a battle knows". These remarks show Lagos jealousy as he try's to tear down Cassio to make himself feel better and look more suitable a lieutenant.