Peirce on Analogy

This paper explores Peirce’s concept of analogy. I begin by arguing that he understands it along two main lines: (1) as a natural cognitive operation that discerns the resemblance of structural relations, pivotally signified by the diagram sign-class, and (2) as a “mixed” form of argument employing abduction, deduction, and induction. After exploring these two aspects, along with their interpenetration, I compare Peirce’s account of analogous reasoning with the highly influential view of the late-Medieval scholastic Thomas Cajetan. I argue that Peirce presents a superior approach because his diagrammatic logic renders a view that is methodologically open to further inquiry, explains that openness in terms of inference through sampling, and capaciously accepts a variety of potential determinations for any one analogy due to the objective vagueness of signs. Cajetan’s appeal to the irreducible proportionality of analogous thinking, on the other hand, excludes further explanation of analogy’s workings.