Knowledge, concept of

5. Defeasibility theories

A basic objection to the foundationalist’s and coherentist’s accounts of justification is that neither seems to be able to show that a true belief which satisfied their accounts would be non-accidentally true. First, as the neighbour/lights case showed, a true belief could be fully justified on their accounts, but not be knowledge. Second, as the Pyrrhonians pointed out, either the beliefs seem to rest upon arbitrary foundations or they seem to be only one of many, equally coherent sets of beliefs. The defeasibility theory was developed, in part, to address these issues. It holds, roughly, that it is not only the evidence that one possesses that makes a belief warranted; it is equally important that there is no defeating evidence that one does not possess. That is, in order for a belief to be warranted it must not only be justified (in the sense required by either the foundationalists or the coherentists) but its justification must be such that there is no truth which, if added to the reasons that justify the belief, is such that the belief would no longer be justified (see Knowledge, defeasibility theory of).

The defeasibility theory can explain why it is not a cognitive accident that the warranted belief is true. If any of the important supporting reasons (those that if removed would destroy the justification) were false, then adding the denial of those reasons (in other words, adding the truth) to one’s beliefs would undermine the justification. In addition, if there is evidence that one does not possess such that it makes it an accident that the belief is true, the propositions describing that evidence would undercut the justification.

A well-known case will help to illustrate this (see Gettier problems). Suppose that I know Tom Grabit well and I see what appears to be Tom stealing a library book: I come to believe that Tom stole a library book. And, let us suppose that Tom did indeed steal the book. Foundationalists and coherentists could deploy their accounts in order to show that the belief is justified. Nevertheless, suppose that, unknown to me, Tom has an identical twin, John, who is a kleptomaniac and was in the library on the day in question and stole a copy of the same book. Even though I arrived at a true belief as a result of good reasoning based upon true propositions, I do not know that Tom stole the book since it is accidental, from the cognitive point of view, that I arrived at the truth. I could just as easily have based my belief on having seen John stealing the book.

The defeasibility theorists would point out that the belief that Tom stole the book is defeated; if the true proposition describing John were added to my beliefs, I would no longer be justified in believing Tom stole the book. In general, the defeasibility theory can rule out accidentally true beliefs as warranted because those beliefs would not be able to stand up to the truth.

Nevertheless, the defeasibility theory has its problems. The primary one is that it seems to exclude too much from what we know. Returning to the Grabit Case, suppose that everything is as it was except that Tom does not have a twin but that Tom’s mother sincerely avows the claims about John. Now, there is a true proposition (Tom’s mother has said sincerely that Tom has an identical twin, John) that defeats the original justification. Hence the belief that Tom stole the book would be defeated. But if Tom’s mother were demented and there never was a twin, it seems that I knew all along that Tom stole the book.

Defeasibility theorists have tried to answer this objection by suggesting ways to distinguish between so-called misleading defeaters (for example, Mrs Grabit sincerely avows that Tom has an identical twin, John) and genuine ones (for example, Tom has an identical twin, John), but there is no agreement among epistemologists that any of these suggestions has succeeded in correctly capturing the distinction between genuine and misleading defeaters.