THE MATERIAL FOR DEVELOPMENT 147 On the psychological side, it is known that to enhance any single quality, it is necessary to isolate the senses as far as possible. A tactile impression is clearer if it is confined to an object which does not conduct heat, that is, which does not at the same time give rise to sensations of temperature, and if the subject stands in .a dark, silent place, free from ocular or auditory impressions which disturb the tactile impressions, the process may be doubled—in the subject isolated from all other impressions arising in the surround- ings; in the material with its system graduated in respect of one quality only. This precision, which serves as the standard of perfection at which we must aim, renders possible a work of internal and external analysis fitted to bring order into the mind of the child. The little child, who is by nature an eager explorer of his •surroundings because he has not yet had time or means for getting to know them intimately, willingly * closes his eyes' or is blind- folded in order to shut out the light when he is exploring shapes with his lands, or willingly accepts darkness in order to listen to slight noises. FUNDAMENTAL QUALITIES COMMON TO EVERYTHING IN THE EDUCATIONAL ENVIRONMENT OF THE CHILD To the above-mentioned characters, others have to be added; these, however, do not refer exclusively to sense-objects, but must be made to include everything which surrounds the child. They are as follows: 1. The Control of Error. In several cases the materials offered to the child involve in themselves the * control of error,* as is instanced by the solid insets; these have wooden bases which are provided with holes into which are fitted cylinders of graduated dimensions, ranging from narrow to wide, or from tall to short, or from small to large. As the hollows correspond exactly to the cylinders to be deposited in them, it is not possible to place them Tvrongly, since at the end there would remain one without a place; this indicates that an error has been made.