Marcel Proust once wrote about a hypothetical sufferer of “spiritual depression” who has no physical incapacity but lacks the will to act. If you've been walled up at home for weeks, you might suffer from this type of mental languor. A good book may be the jolt you need to spur you into new and creative thoughts.
Sometimes your mind wanders, and sometimes your thoughts focus on a specific topic. When your mind wanders, you're not really doing much, but when you focus, you're engaged in a specific mental action; you control what you're thinking about. So what is this mental control?