Nearly everyone has difficulty in the concentration of attention. Brain workers in business and industry, students in high school and college, and even professors in universities, complain of the same difficulty. Attention seems in some way to be at the very core of mental activity, for no matter from what aspect we view the mind, its excellence seems to depend upon the power to concentrate attention. When we examine a growing infant, one of the first signs by which we judge the awakening of intelligence is the power to pay attention or to "notice things." When we examine the intellectual ability of normal adults we do so by means of tests that require close concentration of attention. In judging the intelligence of people with whom we associate every day, we regard one who is able to maintain close attention for long periods of time as a person of strong mind. We rate Thomas Edison as a powerful thinker when we read that he becomes so absorbed in work that he neither eats nor sleeps. Finally, when we examine the insane and the feeble-minded, we find that one form which their derangements take is an inability to control the attention. This evidence, added to our own experience, shows us the importance of concentration of attention in study and we become even more desirous of investigating attention to see how we may develop it. We shall be better able to discuss attention if we select for analysis a concrete situation when the mind is in a state of concentrated attention. Concentrate for a moment upon the letter O. Although you are ostensibly focussing all your powers of attention upon the letter, nevertheless you are really aware of a number of things besides: of other words on the page; of other objects in the field of vision; of sounds in the room and on the street; of sensations from your clothing; and of sensations from your bodily organs, such as the heart and lungs. In addition to these sensations, you will find, if you introspect carefully enough, that your mind also contains a number of ideas and imaginings; thoughts about the paragraph you just read or about one of your lessons. Thus we see that at a time when we apparently focus our attention upon but one thing, we really have a large number of things in our mind, and they are of a great variety. The mental field might be represented by a circle, at the centre of which is the object of attention. It may be an object in the external world perceived through one of the senses, or it may be an idea we are thinking about, such, for example, as the idea of infinity. But whether the thing attended to is a perception or an idea, we may properly speak of it as the object of attention or the "focal" object. In addition to this, we must recognize the presence of a large number of other objects, both sensory and ideational. These are nearer the margin of the mental field, so we call them "marginal."