"Think, professor. Consider everything you know about yourself. Can you believe, for a heartbeat, that I could mark you without your consent?" When Snape did not shoot him down immediately, he continued, "There was more to your summer than rescuing me. You allowed me to change your memories. But they are not erased, just buried. You understand the theory better than I do. But I promise everything will make sense in a moment if you allow me to cast just one spell."