From association with others or from literature and history, ideals begin to be formed which influence conduct. This is brought about largely through the principle of suggestion. In the early years of adolescence children are very susceptible to suggestions, but the suggestive ideas must be introduced by a person who is trusted, admired, or loved, or under circumstances inspiring these feelings; hence the importance to the adolescent of having teachers of strong and inspiring personality. However, if the suggestive idea is to influence action, it must be introduced in such a way as not to set up a reaction against it. Reaction will be set up if the idea is antagonistic to the present ideas, feelings, or aims, or if it is so persistently thrust upon the child that he begins to suspect that he is being unduly influenced. To avoid reaction the parent or teacher should introduce suggestive ideas indirectly. For instance, while the mind is concentrated upon one set of ideas, a suggestive idea that would otherwise be distasteful may be tolerated. It may lie latent for a time, and when it recurs it may be regarded as original, under which condition it is likely to issue in action. The adolescent stage is the period of greatest emotional development, and care should therefore be exercised to have the child's mind dwell upon only those ideas with which worthy emotions are associated. The emotional bent, whether good or bad, is determined to a large extent during this period of adolescence. So far as morality is the subordination of primitive instincts to higher ideas, the child now becomes a moral being. His conduct is now determined by reason and by ideals, and the primitive pleasure-pain motives disappear. It follows that coercion and arbitrary authority have little place in discipline at this period. Social interests are prominent, evidenced by the tendency to co-operate with others for a common end. The games of the period are mainly of the co-operative variety and are marked by a willingness to sacrifice personal interests for the sake of the team, or side. INDIVIDUAL DIFFERENCES While, as noted above, all children have certain common characteristics at each of the three periods of development, it is even more apparent that every child is in many respects different from every other child. He has certain peculiarities that demand particular treatment. It is evident that it would be impossible to enumerate all the individual differences in children. The most that can be done is to classify the most striking differences and endeavour to place individual children in one or other of these classes.