In reconstructing the development of the Treatise, this chapter focuses on the characteristics of Pareto’s sociology and the logical-experimental method he adopted. It highlights how the central theme of his major work revolved around developing the concept of social and political equilibrium. After examining Pareto’s interpretation of historical reality across the various spheres of its composition using a binary scheme of oppositions, it is pointed out how, in his view, the ‘good functioning’ of the social and political system hinges on following the ‘right middle path’—that is, avoiding excesses in one direction or another. The chapter focuses on the dichotomies of logical/non-logical action and logical/non-logical theory, demonstrating how, in Pareto’s view, an imbalance towards either pole destabilises the social system. The chapter concludes with a brief examination of the two primary classes of residues (or psycho-social components) that drive human actions: the instinct for combinations and group-persistences (persistence of aggregates).