-———Illustrasjon: Oda Aurora Norlund———-
By the late nineteenth century logic had undergone a revolution. After almost two millennia of playing around with the limited theory of Aristotelian syllogisms, inference patterns of the form
All Bs are Cs
All As are Bs
Therefore, all As are Cs,
logicians, like Gottlob Frege and Charles Sanders Peirce, greatly advanced their subject. They developed formal languages and theories for the classical propositional calculus and predicate logic of first and second order that greatly outstripped the limits of the old syllogistic approach. Alternative theories of logic were also developed in the decades to follow, such as intuitionistic logic, while some theories fell somewhat out of favor (e.g. second order logic). Underlying logicians’ disagreement about what language to state logical theories in and what principles to count as logical was the question: what is the correct logic?