=Physical Conditions of Attention.=--From what was learned above regarding the relation of nervous energy to active attention, it is evident that the ability to attend to a problem at any given time will depend in part upon the physical condition of the organism. If, therefore, the nervous energy is lowered through fatigue or sickness, the attention will be weakened. For this reason the teaching of subjects, such as arithmetic, grammar, etc., which present difficult problems, and therefore make large demands upon the attention of the scholars, should not be undertaken when the pupils' energy is likely to be at a minimum. Similarly, unsatisfactory conditions in the school-room, such as poor ventilation, uncomfortable seats, excessive heat or cold, all tend to lower the nervous energy and thus prevent a proper concentration of attention upon the regular school work. =Precautions Relating to Voluntary Attention.=--Although voluntary attention is evidently the form of attention possessing real educational value, certain precautions would seem necessary concerning its use. With very young children the aim for attending should evidently not be too remote. In other words, the problem should involve matter in which the children have a direct interest. For this reason it is sometimes said that young children should set their own problems. This is of course a paradox so far as the regular school work is concerned, though it does apply to the pre-school period, and also justifies the claim that with young children the lesson problem should be closely connected with some vital interest. It would be useless, for instance, to try to interest young children in the British North America Act by telling them that the knowledge will be useful when they come to write on their entrance examinations. The story of Sir Isaac Brock, on the other hand, wins attention for itself through the child's patriotism and love of story. Again, the problem demanding attention should not, in the case of young children, be too long or complex. For example, a young child might easily attend to the separate problems of finding out, (1) how many marbles he must have to give four to James and three to William; (2) how many times seven can be taken from twenty-eight; (3) how many marbles James would have if he received four marbles four times; and (4) how many James would have if he received three marbles three times. But if given the problem "to divide twenty-eight marbles between James and William, giving James four every time he gives William three," the problem may be too complex for his present power of attention. A young child has not the control over his knowledge necessary to continue any long process of selecting attention. A relatively short period of attention to any problem, therefore, exhausts the nervous energy in the centres connected with a particular set of experiences. It is for this reason that the lessons in primary classes should be short and varied.