Much ado about contemporary skepticism has to do with a famously concise article Is Justified True Belief Knowledge? by Edmund L. Gettier that appeared in Analysis 23 (1963), pp. 121123.
Thanks to the permission of the author and the efforts of Andrew Chrucky it is also available as a hypertext transcription at:
http://www.ditext.com/gettier/gettier.html
Various attempts have been made in recent years to state necessary and sufficient conditions for someone’s knowing a given proposition. The attempts have often been such that they can be stated in a form similar to the following:
a. S knows that P 
IFF 
 P is true,
 S believes that P, and
 S is justified in believing that P.

For example, Chisholm has held that the following gives the necessary and sufficient conditions for knowledge:
b. S knows that P 
IFF 
 S accepts P,
 S has adequate evidence for P, and
 P is true.

Ayer has stated the necessary and sufficient conditions for knowledge as follows:
c. S knows that P 
IFF 
 P is true,
 S is sure that P is true, and
 S has the right to be sure that P is true.

I shall argue that (a) is false in that the conditions stated therein do not constitute a sufficient condition for the truth of the proposition that S knows that P. The same argument will show that (b) and (c) fail if ‘has adequate evidence for’ or ‘has the right to be sure that’ is substituted for ‘is justified in believing that’ throughout.
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