I play Thoughtseize on my opponent. He reveals two lands and two cards I don't want him to discard. Can I choose to not have him discard anything?

Hi,

That is not possible. Failure to find only applies if you're searching. Thoughtseize does not involve searching, it causes the player to reveal their hand and you to choose a nonland card. If there's a nonland card in their hand, you have to choose it, even if you don't want them to discard it.

Nathan Long
Magic Rules NetRep