Again, the thought may be difficult to follow because your previous knowledge is deficient; perhaps the discussion involves some fact which you never did comprehend clearly, and you will naturally fail to understand something built upon it. If deficiency of knowledge is the cause of your lapses of attention, the obvious remedy is to turn back and study the fundamental facts; to lay a firm foundation in your subjects of study. This discussion shows that the conditions at time of concentrated attention are very complex; that the mind is full of a number of things; that your object as a student is to keep some one thing at the focus of your mind, and that in doing so you must continuously ignore other mental contents. In our psychological descriptions we have implied that the mind stands still at times, permitting us to take a cross-section and examine it minutely. As a matter of fact, the mind never stands still. It continually moves along, and at no two moments is it exactly the same. This results in a condition whereby an idea which is at one moment at the centre cannot remain there unless it takes on a slightly different appearance from moment to moment. When you attempted to fix your attention upon the letter O, you found a constant tendency to shift the attention, perhaps to a variation in the intensity of the type or to a flaw in the type or in the paper. In view of the inevitable nature of these changes, you see that in spite of your best efforts you cannot expect to maintain any object of study inflexibly at the centre of attention. The way to do is to manipulate the object so that it will appear from moment to moment in a slightly different light. If, for example, you are trying to concentrate upon a rule of English grammar long enough to memorize it, do not read it over and over again, depending solely upon repetition. A better way, after thoroughly comprehending it, is to think about it in several relations; compare it with other rules, noting points of likeness and difference; apply it to the construction of a sentence. The essential thing is to do something with it. Only thus can you keep it in the focus of attention. This is equivalent to the restatement of another fact stressed in a previous chapter, namely, that the mind is not a passive thing that stands still, but an active thing. When you give attention, you actively select from a number of possible objects one to be clearer than the rest. This selection requires effort under most conditions of study, but you may be cheered by the thought that as you develop interest in the fields of study, and as you develop habits of ignoring distractions, you will be able to fixate your attention with less and less effort. A further important fact is that as you develop power to select objects for the consideration of attention, you develop simultaneously other mental processes--the ability to memorize, to economize time and effort and to control future thoughts and actions.