Formal Logic, Informal Reasoning, and Their Disputable Relevance

Abstract

Our commonplace practices of reasoning in natural languages-i.e. informal reasoning- are often believed to satisfactorily map on a formal system of logic-i.e. formal logic- which typically consists of formal syntax, formal semantics, and related proof system. Nonetheless, it then becomes significant to ask on what grounds formal logic legitimately has such relevance to informal reasoning. On one standard view, formal logic governs all rationally admissible tokens of informal reasoning. This view – either in its descriptive or normative construal - has been seriously challenged by Gilbert Harman and Peter Wason who commonly hold that formal logic has no (descriptive/normative) relevance to informal reasoning. This paper provides a critical survey of these distinct accounts, narrowing down the scope of their common thesis based on the possibility of logical pluralism. As it argues, a singular formal system of proof such as classical first-order deductive logic cannot single-handedly satisfy the standard view.