Robert Greene's framing that self-mastery and craft-mastery are not two separate pursuits but "an organic whole" — the dividing line between them is, in his words, "just a matter of words." Greene states it directly: "yourself is the craft and the craft is yourself," and describes his own life's outcome as "mastery of self through craft" rather than self-mastery achieved independently of any concrete discipline.
The mechanism: a craft's inherent limitations — e.g. having to learn scales before playing piano — are exactly what train and develop the person practicing it. Self-mastery pursued in the abstract, disconnected from a concrete goal or discipline, "won't mean anything" by Greene's account; the constraints are generative, not merely restrictive. This connects to Discovering Your Life's Task (Vocation) as the phase that identifies which craft is the right vehicle for this fusion, and stands as a reframe of craft limitations as developmental substance rather than obstacles to work around.