Our attention is guided by interest. If a man merely allows his thoughts to flow at random, thinking only of those things which spontaneously arouse his interest, he may or may not attend to things worth thinking about. All will depend upon the path in which his natural interests run. But the point is that if the subject he thinks about is valuable, it will be so only by accident; whether or not his thinking is useful will depend upon mere chance. If however he consciously chooses a subject—chooses it because he believes it to be important—then his thinking will be worth while. But there is another reason why concentration is necessary. Suppose a man started to put up a barbed wire fence, got as far as driving in all the posts, then lost interest in the fences and decided to grow potatoes in his field, plowed up the ground, lost interest in the field and neglected to plant the seeds; decided to paint his house, got the porch done, lost interest . . . That man might work as hard as any other man, but he would never get anything done. So with the mind wanderer and the concentrator. The mind wanderer thinks of a problem, loses interest, and abandons it. The concentrator sticks to it until it is solved. Much of our mind wandering is due to the fact that we are not fully convinced of the importance of the problem being attacked, or that we regard other problems or ideas as more important. Concentration consists in devoting one’s mind to the solution of one problem. During our train of thought associations bring up new ideas or suggest problems which do not bear on the question at hand. Now when we wander, when we follow up these irrelevant ideas or suggested problems, or when we happen to glance at something or hear something and begin to think of that, we do so because of a half-conscious belief that the new idea, problem or fact needs attending to, is important. I have already pointed out that if this new idea is important it will be so only by accident. If we were consciously to ask ourselves whether any of these irrelevant problems were as important as the one we were concentrating on, or even important at all, we would find, nine times out of ten, that they were not. Therefore before beginning to concentrate you should assure yourself that the problem you are about to attack is one worth solving, or at least devoting a certain time to. And during that time you should think only of that problem, and unhesitatingly throw out all irrelevant suggestions coming either from your course of thought or from external sights and sounds.