UCalgary Peripatetic Seminar: Xuefeng Wen

Slightly altering and extending McGee’s semantics for conditionals, we define a ternary notion of logical consequence for the validity of natural language arguments. The ternary logical consequence can be regarded as a unification of two kinds of validity in the literature. By the new notion of logical consequence, an inference is not just valid or invalid, but valid or invalid under a set of assumptions. Based on this notion, we give a unified solution to some typical puzzles concerning conditionals and epistemic modals, including the (in)validity of modus ponens, modus tollens, Import-Export, conditional excluded middle, Or-to-If, and fatalism arguments, as well as the puzzle of Moore sentences and the scope ambiguity problem in modal conditionals.