The modern ideal of international cooperation, which it is desired to incorporate in the future education of the young, is based on the two main principles of solidarity and justice. In this essay, the author discusses two aspects on the problem of solidarity: (1) moral aspect; and (2) intellectual aspect. In connection with the moral aspect, the author briefly considers the structure of child societies with special reference to the development of the idea of rules. For the intellectual aspect, he examines the question of the exchange of ideas and discussion among children. By these two converging Methods he arrives at the conclusion that the "active" methods of self-government and collective work offer the best means of cultivating the spirit of intellectual and moral solidarity in children. (Contains 2 footnotes.)