But because these are the mental phase of behavior, the needed play of individuality--or freedom--cannot be separated from opportunity for free play of physical movements. Enforced physical quietude may be unfavorable to realization of a problem, to undertaking the observations needed to define it, and to performance of the experiments which test the ideas suggested. Much has been said about the importance of "self-activity" in education, but the conception has too frequently been restricted to something merely internal--something excluding the free use of sensory and motor organs. Those who are at the stage of learning from symbols, or who are engaged in elaborating the implications of a problem or idea preliminary to more carefully thought-out activity, may need little perceptible overt activity. But the whole cycle of self-activity demands an opportunity for investigation and experimentation, for trying out one's ideas upon things, discovering what can be done with materials and appliances. And this is incompatible with closely restricted physical activity. Individual activity has sometimes been taken as meaning leaving a pupil to work by himself or alone. Relief from need of attending to what any one else is doing is truly required to secure calm and concentration. Children, like grown persons, require a judicious amount of being let alone. But the time, place, and amount of such separate work is a matter of detail, not of principle. There is no inherent opposition between working with others and working as an individual. On the contrary, certain capacities of an individual are not brought out except under the stimulus of associating with others. That a child must work alone and not engage in group activities in order to be free and let his individuality develop, is a notion which measures individuality by spatial distance and makes a physical thing of it.