Interest and self-interest are concepts that have attracted multiple interdisciplinary approaches in the last decades. In this monograph, the author examines anew all nomenclature of these concepts in ancient Greek, and recapitulates relevant previous debates. This survey enables him to deal with major issues related to the Greek citizen and the polis as a whole: the gnoseology of self-interest, the manipulation of conflicting interests, the balance between expediency and justice, the vigour of competitive spirit among the Greeks and the conciliation of private with common good. The sympheron cannot be perceived beyond the context and framework of the much-discussed topics of individualism and utilitarian trends in ancient Greek thought and civic ideology. Exploring these aspects of his subject-matter, the author provides a number of clues as to how one may better comprehend the polis stratagems to invent those devices needed to aggregate the one into the many.