There was a long tradition in philosophy according to which good reasoning had to be deductively valid. However, that tradition began to be questioned in the 1960’s, and is now thoroughly discredited. What caused its downfall was the recognition that many familiar kinds of reasoning are not deductively valid, but clearly confer justification on their conclusions. Here are some simple examples.

Pollock is developing, and testing computationally a theory of rationality. I endorse his claim that the structure of argument is not always linear, his replacement of the deductive-inductive distinction with a deductive-defeasible distinction, and hi s distinction between rebutting defeaters, and undercutting defeaters. I question the absence of pragmatic constraints in his acceptance rule, an absence which fits ill with his criterion of interruptibility, that it is reasonable to act on the conclusions drawn at any point. I also wonder about (...) building a model of a rational agent in which there is no place for interpersonal argumentative discussion. (shrink)