The Indefeasibility of Abduction

It is often assumed that (rationally compelling, good) reasoning is defeasible if and only if it relies on an invalid argument, and that reasoning is indefeasible if and only if it relies only on valid arguments. We here argue that this common assumption is incorrect. We first argue that this equivalence should not be treated as definitional; we then argue that some reasoning is defeasible despite relying only on valid arguments, while other reasoning is indefeasible despite relying on invalid arguments. More specifically, we argue that deduction’s being subject to rebutting defeaters renders it defeasible, despite its relying only on valid arguments, while abduction’s being immune to rebutting defeaters renders it indefeasible, despite its relying on invalid arguments. We then offer an explanatory diagnosis of the errant assumption at issue, suggesting that it reflects both a too-quick generalization from the case of (enumerative) induction and a general tendency to conflate metaphysical and epistemic notions. Finally, we suggest that a kind of reasoning is an ultimate arbiter of disputes in a domain if and only if it is indefeasible in that domain, so that abduction is an ultimate arbiter of disputes in any domain in which it is operative.