Full Idea
In general a knowledge claim is legitimate only if all relevant defeaters have been eliminated.
Clarification
A 'defeator' would undermine someone's claim to know something
Gist of Idea
Knowledge is legitimate only if all relevant defeaters have been eliminated
Source
Robert Fogelin (Walking the Tightrope of Reason [2003], Ch.4)
Book Reference
Fogelin,Robert: 'Walking the Tightrope of Reason' [OUP 2004], p.102
A Reaction
The problem here is what is 'relevant'. Fogelin's example is 'Are you sure the suspect doesn't have a twin brother?' If virtual reality is relevant, most knowledge is defeated. Certainly, imaginative people feel that they know less than others.