It may be imagined that if we think too much we will be liable permanently to injure our mighty intellects. This has sometimes happened. But there is no serious danger of it. Thinking on one useful subject for a long while will not hurt you any more than thinking on a thousand different useless subjects for the same period. But of course you should not try to concentrate when you are sleepy, when you have a headache, when some other bodily pain distracts your attention, or when your mind is in any way tired. If you attempt to concentrate at these times you will endanger your mental and physical health. Not only this, but the thinking done during such periods will be of such poor quality that it will be practically useless if not harmful. This applies even to cases where mental fatigue is almost inappreciable. Thinking done in the evening seldom approaches in efficacy the thinking done in the first hours of the morning. But you should always make sure your mind is actually tired. It may merely be tired of a particular subject. An objection of a different kind may be raised against concentrating at every opportunity. It has often been noticed that names have been recalled and problems solved when we were thinking of something else. It may be urged that such solutions would not have occurred when concentrating, because the exact associations which led up to them would not have been present. This is occasionally true. But there are still reasons why I must maintain my position. No matter how well a man may have trained himself to concentrate, there will always be short periods when his mind will wander, and these will suffice for any accidental associations. Moreover, the fact that these mind wandering periods _occasionally_ do good does not excuse their existence. The most fallacious ideas, the most demoniacal practices, the most despicable characters of history, have _occasionally_ done good. The fact is that for every useful association which occurs during mind wandering, ten associations just as useful will occur during concentration. The only reason useful mind wandering associations appear frequent is that they are unexpected, therefore more noticed when they come.