It is probably the internal perception of this development which makes the exercise pleasing, and induces prolonged application to the same task. To quench thirst, it is not sufficient to see or to sip water; the thirsty man must drink his fill: that is to say, must take in the quantity his organism requires; so, to satisfy this kind of psychical hunger and thirst, it is not sufficient to see things cursorily, much less "to hear them described"; it is necessary to possess them and to use them to the full for the satisfaction of the needs of the inner life. This fact stands revealed as the basis of all psychical construction, and the sole secret of education. The external object is the gymnasium on which the spirit exercises itself, and such "internal" exercises are primarily "in themselves" the end and aim of action. Hence the solid insets are not intended to give the child a knowledge of dimensions, nor are the plane insets designed to give him a conception of forms; the purpose of these, as of all the other objects, is to make the child exercise his activities. The fact that the child really acquires by these means definite knowledge, the recollection of which is vivid in proportion to the fixity and intensity of his attention, is a necessary result; and, indeed, it is precisely the sensory knowledge of dimensions, forms and colors, etc., thus acquired, which makes the continuation of such internal exercises in fields progressively vaster and higher, a possible achievement. Hitherto, all psychologists have agreed that instability of attention is the characteristic of little children of three or four years old; attracted by everything they see, they pass from object to object, unable to concentrate on any; and generally the difficulty of fixing the attention of children is the stumbling-block of their education, William James speaks of "that extreme mobility of the attention with which we are all familiar in children, and which makes their first lessons such rough affairs.... The reflex and passive character of the attention ... which makes the child seem to belong less to himself than to every object which happens to catch his notice, is the first thing which the teacher must overcome.... The faculty of voluntarily bringing back a wandering attention, over and over again is the very root of judgment, character and will.... An education which should improve this faculty would be _the_ education _par excellence_." Thus man, acting by himself alone, never successfully arrests and fixes that _inquiring_ attention which wanders from object to object.