DUKE Where your good word cannot advantage him
Your slander never can endamage him.
Therefore, the office is indifferent,
Being entreated to it by your friend.
PROTEUS You have prevailed, my lord. If I can do it
By aught that I can speak in his dispraise
She shall not long continue love to him.
But say this weed her love from Valentine,
It follows not that she will love Sir Thurio. (3.2.42-50)
The Duke is as given to special pleading as is Proteus: where your good word cannot advantage him—because Valentine’s already an outcast, banished, and there’s nothing that Proteus could say that would change that, bring him any benefit—your slander never can endamage him. He’s at rock bottom already, he’s finished, and so nothing that you say (even if it’s false, apparently) can damage him further. Nice. And therefore the office is indifferent, it makes no difference either way what you do; in effect it’s morally neutral! It’s me asking you, after all; you’re being entreated to it by your friend. I’m your friend now. (So the Duke refers to himself in the same intimate position with Proteus as Valentine has been, as if having superseded him in affection, despite their difference in rank. He gets Proteus’s class insecurity at least, even if he’s unaware of the other things that are going on here.)
Proteus doesn’t need much (any) persuasion: you have prevailed, my lord. I’m won over, much as it pains me etc etc. And if I can do it by aught that I can speak in his dispraise—if anything I say to disparage Valentine manages to put her off him—she shall not long continue love to him. She’ll change her mind soon enough. But—a thought apparently occurs. (It doesn’t; this is Proteus’s chief concern now.) Say this weed her love from Valentine, uproot her previous attachment from him (as if it were almost nothing, noxious, unprofitable, unwelcome): it follows not that she will love Sir Thurio. Just because she stops loving Valentine—thanks to my comprehensive wrecking of his reputation in her eyes—it doesn’t mean that she’ll automatically fall for Sir Thurio. Hmmmm? She might fall for someone else…
And still no one’s mentioning Silvia by name.