Diagrammatology investigates the role of diagrams for thought and knowledge. Based on the general doctrine of diagrams in Charles Peirce's mature work, Diagrammatology claims diagrams to constitute a centerpiece of epistemology. This book reflects Peirce's work on the issue in Husserl's contemporaneous doctrine of categorical intuition and charts the many unnoticed similarities between Peircean semiotics and early Husserlian phenomenology.