The talk will present Peirce’s notion of diagrammatical reasoning. The core idea is that all deductive reasoning takes place by means of diagrams, explicitly or implicitly. Everyday examples include the use of maps, schemata, charts, pictures, gestures, language; scientific examples include reasoning with specially constructed diagrams, algebras, formalisms, etc. An important implication is that reasoning also involves a perceptual-observational aspect. Reasoning takes you from one proposition to another – so another implication is that diagrams and pictures may be used in the expression of propositions. Peirce’s functional definition of propositions thus extend their scope beyond human language, to non-linguistic semiotics as well as animal communication.