Mastery I believe occurs when you no longer need to think about what you're doing. In the 4 stages of learning, it's the stage of unconscious competence. When you're beginning the journey in learning something, you're at a stage of unconscious incompetence, because you don't know what you don't know. As you start to learn new behaviours, it can be quite overwhelming - this is the stage of conscious incompetence, you now know what you don't know. As you continue to practice, you move into the stage of conscious competence, you know what you're doing and you're becoming better at it but you still need to work on it. And after years of practice, it becomes a habit that you don't think about, which is the stage of unconscious competence - ie. mastery. What skill have you reached a level of mastery with?