Spinoza, Peirce and Deleuze are, in different ways, philosophers of immanence. Rocco Gangle addresses the methodological questions raised by a commitment to immanence in terms of how diagrams may be used both as tools and as objects of philosophical investigation. Gangle integrates insights from Spinozist metaphysics, Peircean semiotics and Deleuze's philosophy of difference in conjunction with the formal operations of category theory. He introduces the methods of category theory from a philosophical and diagrammatic perspective in a way that will allow philosophers with little or no mathematical training to come to grips with this important field.