This article demonstrates that arguments which historians use can be expressed in terms of formal logic to revealing effect. It is widely taken for granted and sometimes explicitly stated that historical inference is not susceptible of being formalized, at least not in a way that might add something to historians’ understanding of the logic of their reasoning from evidence. The two model derivations in formal logic included here show otherwise. Each is a representation in propositional logic of an historical argument (...) relating to the evidence of medieval marriage law, annulment processes and papal dispensations. The way in which the conclusion is derived from the premises in each case permits the formal logical validity of inferences made to be traced and checked. The paper aims to make the logic underlying much historical argumentation more transparent. A supplementary aim is to show that this kind of historical reasoning is compatible with Bayesian logic. (shrink)