This book explores the metaphysics of political communities. It discusses how and why a plurality of individuals becomes a political unity, what principles or forces keep that unity together, and what threats that unity can be faced with. In Part I, the author justifies the need for the notion of substance in metaphysics in general and in the metaphysics of politics in particular. He spells out a moderately realist theory of substances and of their principles of unity, which supports substantial gradualism. Part II concerns action theory and the nature of practical reason. The author...