The present book is a natural outgrowth of Rescher’s longstanding preoccupation with the rational systematization of our knowledge as manifested in such earlier works as Cognitive Systematization (Oxford: Blackwell, 1979), and Complexity (New Brunswick: Transaction Publishers, 1998). Accordingly, the role of principles in human affairs is crucial and ubiquitous. Principology, the theory of principles—underdeveloped through it may be—is accordingly bound to find a significant place in the sphere of philosophical inquiry regarding matters of thought and action.