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  • Print publication year: 2008
  • Online publication date: June 2012

23 - Defeasible Reasoning


The Inadequacy of Deductive Reasoning

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 1960s, 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:


Most of our knowledge of the world derives from some form of perception. But clearly, perception is fallible. For instance, I may believe that the wall is gray on the basis of its looking gray to me. But it may actually be white, and it only looks gray because it is dimly illuminated. In this example, my evidence (the wall's looking gray) makes it reasonable for me to conclude that the wall is gray, but further evidence could force me to retract that conclusion. Such a conclusion is said to be justified defeasibly, and the considerations that would make it unjustified are defeaters.


There is one kind of reasoning that few ever supposed to be deductive, but it was often conveniently ignored when claiming that good reasoning had to be deductive. This is inductive reasoning, where we generalize from a restricted sample to an unrestrictedly general conclusion. For example, having observed a number of mammals and noted that they were all warm-blooded, biologists concluded that all mammals are warm-blooded. Hume's concern with induction was just that it is not deductive.

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