Musashi's maxim isn't a cheerful plea to forget your mistakes—it's far sharper than that. He's distinguishing between regretting the act itself (which poisons your judgment going forward) and learning from it (which improves your next move). A chess player who torments herself over a blunder three games ago doesn't play better; she plays tentatively, second-guessing sound moves because fear has clouded her thinking. The swordmaster knew that regret is less about moral correction and more about paralysis, and paralysis gets you killed—whether you're wielding a blade or simply trying to live with clarity.