 While the first contribution, that is, an extension from monadic to predicate logic, has positioned Peirce as a founder of modern logic along with Frege, it took much longer for Peirce’s other achievements to receive proper attention from logicians or philosophers. The entry aims to draw a road map for Peirce’s journey in deductive logic so that one may realize his accomplishments are very much connected with each other. More specifically, Peirce’s achievements in deductive logic were accumulative. After being able to formalize polyadic relations with new symbolic notation, Peirce devised a totally new form of representation—diagrammatic systems. What we can formalize is extended and how we can formalize what we can formalize is extended. And Peirce ventured into what our formalization represents and suggested a more fine-grained or a bigger territory of semantic values than binary True or False values.