Even in the first chapter I intimated that logic would constitute a part of the science of thinking. I intimated, moreover, that it would constitute almost the whole of what may be called the negative side of thinking—those rules which serve to steer thought aright. Though cautionary, the advice given in this chapter is not usually given in books on logic. But though I cannot overemphasize the importance of a knowledge of logic, I cannot deal with it here. The science can receive justice only in a book devoted entirely to it. If he has not already done so the would-be thinker should study a work on logic, for unless the present book is supplemented by some treatise on that science it cannot be regarded as complete. In order not to confuse the reader I shall recommend only one book. In order to encourage him I shall recommend a small book, one not so deep as to be incomprehensible or repulsive to the beginner, but at the same time one which is recognized as a standard treatise:—_Elementary Lessons in Logic_, by Stanley Jevons. IV CONCENTRATION What is the hardest task in the world? To think.—EMERSON. We have been dealing with the subject of thinking. We have considered it from both a positive and negative side. But while we have devoted our attention to thinking, we have neglected the thinker. In more scientific terms, we have treated thought from the logical side; we are now to treat it from the psychological. Few people will admit specific faults in themselves of any kind, especially if these happen to be intellectual. But almost any man is willing to confess that he cannot always “concentrate” when he wants to, in fact, that he is one of the countless victims of “mind wandering.” Most of us imagine we know just what we mean by both these terms. But if we are to judge by most of what has been written, no two terms are more misconceived. Before trying to find the best means of concentrating, we must first find just what we mean by concentration. In a previous chapter I said that suggestions for solutions “occurred.” I did not say how or why. To discover this we must refer to the famous psychological principle of association. Any train of thought is made possible by previous connections of ideas in our minds. While a girl sits at her window a parade passes along a nearby street. The band is playing, and ere the tune is completed the band has gone so far that the music is no longer audible. But the tune still goes along in her mind, and she completes it herself. It suggests a dance she had been to where it was played, and this suggests that she danced the two-step to it. The two-step suggests the more modern one-step, and this leads her to compare the familiar dancing of to-day with the distant and respectful minuet.