To understand what an interest is we should picture the mind as a system of very delicately poised balances, a system which so long as we are in health is constantly growing. Every situation we come into disturbs some of these balances to some degree. The ways in which they swing back to a new equipoise are the impulses with which we respond to the situation. And the chief balances in the system are our chief interests. Suppose that we carry a magnetic compass about in the neighbourhood of powerful magnets. The needle waggles as we move and comes to rest pointing in a new direction whenever we stand still in a new position. Suppose that instead of a single compass we carry an arrangement of many magnetic needles, large and small, swung so that they influence one another, some able only to swing horizontally, others vertically, others hung freely. As we move, the perturbations in this system will be very complicated. But for every position in which we place it there will be a final position of rest for all the needles into which they will in the end settle down, a general poise for the whole system. But even a slight displacement may set the whole assemblage of needles busily readjusting themselves. One further complication. Suppose that while all the needles influence one another, some of them respond only to some of the outer magnets among which the system is moving. The reader can easily draw a diagram if his imagination needs a visual support. The mind is not unlike such a system if we imagine it to be incredibly complex. The needles are our interests, varying in their importance, that is in the degree to which any movement they make involves movement in the other needles. Each new disequilibrium, which a shift of position, a fresh situation, entails, corresponds to a need; and the wagglings which ensue as the system rearranges itself are our responses, the impulses through which we seek to meet the need. Often the new poise is not found until long after the original disturbance. Thus states of strain can arise which last for years. The child comes into the world as a comparatively simple arrangement. Few things affect him, comparatively speaking, and his responses also are few and simple, but he very quickly becomes more complicated. His recurrent needs for food and for various attentions are constantly setting all his needles swinging. Little by little separate needs become departmentalized as it were, sub-systems are formed; hunger causes one set of responses, the sight of his toys another, loud noises yet another, and so on. But the sub-systems never become quite independent. So he grows up, becoming susceptible to ever more numerous and more delicate influences.