From Hegel to the present, the humanities and social sciences have revealed the volatile power of third agency. Systems of thought and practice are often disturbed by the presence of a figure that exceeds traditional binary oppositions. The articles in this volume trace the role of these triadic figures across a broad range of discourses in social theory, philosophy and science studies. Modernity emerges as a mode of system-formation, perpetuation and self-reflection that is deeply rooted in the dynamics of dialectic and paradox. The volume offers an approach which is both systematic and genealogical, providing innovative perspectives on such major thinkers as Adorno, Agamben, Derrida, C. S. Peirce, the Romantics and Simmel as well as phenomena like the psychology of jealousy and envy, the epistemic status of scientific images and conceptions of metabolism. It is the first attempt to look at configurations of the third as a paradigm for the 'unfinished project of modernity' (Habermas).