Defeasibility Condition

Lehrer and Paxson put forth a simpler requirement. They state that if there is no other truth (Y) such that person A's believing it would have destroyed his justification for believing that X, then person A does know that X. This may be, in modern times, dubbed the KISS rule; or keep it simple stupid.

This requirement to the three conditions of justified true belief tends to appeal to the common sense side. Mrs. Knull can then be said to know that her secretary is in the office, because there is no other truth that would have destroyed her justification for the secretary's presence in the office.

This condition seems to be the one which may best support the counter-examples offered by Gettier. In reality, we cannot go about in doubt of everything that we know. Much like a justified true belief of the sun's revolution about the Earth, we may say that we know that the universe is geocentric, until we have another truth (heliocentric) which destroys this view.