<Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (November 2015) (Learn how and when to remove this template message) </Td> </Tr> <P> The responses to Gettier have been varied . Usually, they have involved substantial attempts to provide a definition of knowledge different from the classical one, either by recasting knowledge as justified true belief with some additional fourth condition, or proposing a completely new set of conditions, disregarding the classical ones entirely . </P> <P> In one response to Gettier, the American philosopher Richard Kirkham has argued that the only definition of knowledge that could ever be immune to all counterexamples is the infallibilist one . To qualify as an item of knowledge, goes the theory, a belief must not only be true and justified, the justification of the belief must necessitate its truth . In other words, the justification for the belief must be infallible . </P> <P> Yet another possible candidate for the fourth condition of knowledge is indefeasibility . Defeasibility theory maintains that there should be no overriding or defeating truths for the reasons that justify one's belief . For example, suppose that person S believes he saw Tom Grabit steal a book from the library and uses this to justify the claim that Tom Grabit stole a book from the library . A possible defeater or overriding proposition for such a claim could be a true proposition like, "Tom Grabit's identical twin Sam is currently in the same town as Tom ." When no defeaters of one's justification exist, a subject would be epistemologically justified . </P>

What does it mean to know something philosophy