The reply is, by exercise; mental strength is secured in exactly the same way that physical strength is secured, by exercise. We think something, perhaps with difficulty the first time; we think the same thing again, and it becomes easier this time; we think it again and again; it then becomes a mental habit. We continue to think the same thing; finally it becomes automatic; we can no longer help thinking this thing; we are now positive of what we think; there is no longer any doubt about it. We are sure; we know.
