Brandom's Account of Defeasible Reasoning: Problems and Solutions
Robert Brandom has provided what is probably one of the best worked out accounts of how the meanings of linguistic expressions are determined by how they are used—in particular, used in inferences. There are three different types of inferential relations in terms of which Brandom gives his account: commitment-preserving, entitlement-preserving, and incompatibility relations. Brandom also recognizes that most of the reasoning we engage in is defeasible (or deductively inconclusive). For example, the inference from ‘Tweety is a bird’ to ‘Tweety can fly’ is defeasible, because it can be defeated if there is stronger overriding reason to deny that ‘Tweety can fly’—such as Tweety’s being a penguin. Surprisingly, Brandom’s three types of inferential relations are inadequate for describing defeasible inference. In my dissertation I explain how the problem arises—it’s actually two problems—and I propose a solution that is consistent with Brandom’s overall approach. The first problem is that although Brandom's account does explain how someone can lose entitlement to a claim by committing themselves to some other claim, as in the Tweety example, it doesn’t allow subsequent recovery of entitlement to that claim by the addition of yet further information—say, that Tweety is a penguin with a jetpack. Once defeated (by some information), an inference stays defeated, on Brandom's account. The second problem is that of interpretation: when should we interpret someone as committed to the propriety of an inference that is defeasible? Brandom's account of what it is to endorse an inferential relation has no room for the important distinction between endorsing an inference in a context in which it happens to be defeated, and not endorsing it at all. In the latter portion of this dissertation I propose various modifications to Brandom’s account that will allow it overcome these problems. I solve the first problem by modifying Brandom’s account of how someone is obliged to update their beliefs in light of the inferential relations they endorse. I solve the second problem by modifying Brandom’s account of when we can appropriately interpret someone as endorsing particular inferential relations.