In analyzing this ‘many-sided interest,’ Herbart holds that ideas and interests spring from two main sources,--‘experience,’ which furnishes us with a knowledge of nature, and ‘social intercourse,’ from which come the sentiments toward our fellowmen. Interests may, therefore, be classed as belonging to (1) ‘knowledge’ or to (2) ‘participation.’ These two sets of interests, in turn, Herbart divides into three groups each. He classed the ‘knowledge’ interests as (a) ‘empirical,’ appealing directly to the senses; (b) ‘speculative,’ seeking to perceive the relations of cause and effect; and (c) ‘æsthetic,’ resting upon the enjoyment of contemplation. The ‘participation’ interests are divided into (a) ‘sympathetic,’ dealing with relations to other individuals; (b) ‘social,’ including the community as a whole; and (c) ‘religious,’ treating one’s relations to the Divine. Instruction must, therefore, develop all these interests, and, to correspond with the two main groups, Herbart divides all studies into two branches,--the (1) ‘historical,’ including history, literature, and languages; and the (2) ‘scientific,’ embracing mathematics, as well as the natural sciences. Although recognizing the value of both groups, Herbart especially stressed the ‘historical,’ on the ground that history and literature are of greater importance as the sources of moral ideas and sentiments. [Sidenote: ‘Correlation’ and ‘concentration.’] [Sidenote: ‘Culture epochs.’] But, while all the subjects, ‘historical’ and ‘scientific,’ are needed for a ‘many-sided interest,’ and the various studies have for convenience been separated and classified by themselves, Herbart holds that they must be so arranged in the curriculum as to become unified and an organic whole, if the unity of the pupil’s consciousness is to be maintained. This position forecasts the emphasis upon ‘correlation,’ or the unification of studies, so common among his followers. The principle was further developed by later Herbartians under the name of ‘concentration,’ or the unifying of all subjects around one or two common central studies, such as literature or history. But the selection and articulation of the subject-matter in such a way as to arouse many-sidedness and harmony is not more than hinted at by Herbart himself. He specifically holds, however, that the _Odyssey_ should be the first work read, since this represents the interests and activities of the race while in its youth, and would appeal to the individual during the same stage. He would follow this with other Greek classics in the order of the growing complexity of racial interests depicted in them. This tentative endeavor of Herbart, in the selection of material for the course of study, to parallel the development of the individual with that of the race, was continued and enlarged by his disciples. It became especially definite and fixed in the ‘culture epochs’ theory formulated by Ziller and others.