Part of the problem with putting some of these ideas into practice is a disconnect between what we think of when we talk of mastery and the more open kind of learning that Kahn proposes. Mastery is demonstrating knowledge (usually recall) of set lists of facts, concepts, and procedures, (“You either know a concept or you don’t.”) But if students are exploring a topic in depth, mastery may not be the issue. Sure the student has to understand certain concepts, gather certain facts, and perhaps explore certain procedures to reach a deep understanding of the topic. But the purpose isn’t mastery of the concepts within a topic but deep understanding of the topic itself. If a student discovers he misunderstands a concept, he needs to go back and figure it out, just as he might in mastery learning. But the impetus for the student is not to pass the test on the concept, but to understand it in order to learn more.