“When a person is not doing his job, there can only be two reasons for it. The person either can't do it or won't do it; he is either not capable or not motivated.”
Can't or Won't
From High Output Management (Random House, 1983), p. 157, in Chapter 11 (“The Sports Analogy”). The line sets up Grove’s argument that a manager has exactly two levers for raising an individual’s output (motivation and training), so the can’t/won’t question tells you which one is available. Grove gives the working test on the same page: if the person’s life depended on doing the work, could he do it? If yes, the problem is motivation. If no, it’s capability. His own examples are the violin (couldn’t, even for his life) and a six-minute mile (probably could, if forced).
The practical handle is that the wrong diagnosis wastes the wrong lever. A capability problem treated as a motivation problem produces pressure the person can’t act on. A motivation problem treated as a capability problem produces training the person doesn’t need.