‘Men should be what they seem.’

On its surface this is a plain moral: a man’s outside should match his inside. People who are not what they appear to be should not be allowed to look like honest men. That is what ‘would they might seem none’ means: if only the fakes looked like nothing at all.

From Iago, the sentiment is poison dressed as proverb. He is talking about Cassio, and the trick is in what he refuses to say. Solemnly agreeing that honest-looking men should be honest plants a thought: maybe Cassio only looks it.

Othello takes the bait and repeats the line back: ‘Certain, men should be what they seem.’ Iago then closes the trap: ‘Why, then, I think Cassio’s an honest man.’ That one word, think, does all the damage.

This exchange comes early in Act 3 Scene 3, the long scene in which Iago moves Othello from settled love to murderous jealousy in a single conversation, without producing one piece of real evidence.

A few lines later in the same scene comes the most famous warning in the play: jealousy as the green-eyed monster.

Quote sites regularly attribute this line to Othello. He does say it, but as an echo. Iago speaks the famous phrasing first; Othello repeats it back a moment later as ‘Certain, men should be what they seem.’ Both versions sit in the same exchange in Act 3 Scene 3.