"Let me tell you a little secret. A man’s ability as a swordsman of the other kind, to borrow your words, hinges much upon his health of body and peace of mind. If you’d fain cause his—ah—resolution to droop, you have but oft to berate him in harsh and wounding terms. If you’re fain to have him serve you with vigor, flatter and praise him; make him think himself worthier than in his heart he knows himself to be."
January 1, 1970
https://en.wikiquote.org/wiki/L._Sprague_de_Camp