Charles Sanders Peirce developed an optical system for the logic of relations based on the concept of graph, a tool able to produce models representing states of things and to develop new models that are closer to intuition. The thesis that we want to defend in this paper is that Peircean graphs form part of diagrammatic thought, a veritable logic of the concept that can justifably be placed in correspondence with the geometrization of meaning implemented by René Thom.