Knowledge, Analysis of

Knowledge as Justified True Belief

An intuitively plausible analysis of what it means to say that someone knows something (a proposition) specifies three conditions that must be met: (1) the person must believe the proposition, (2) the proposition must be true, and (3) the person must be justified in believing the proposition (i.e., he or she must believe it for good reasons). Saying that a person knows something implies that these three conditions are met. The analysis also asserts the converse, that meeting the three conditions implies that the person knows the proposition.

This is referred to as the justified true belief (JTB) analysis of propositional knowledge. The rationale for the belief and truth condition seems obvious. It would be contradictory to say both that Jeff knows that the earth revolves around the sun and that Jeff does not believe that the earth revolves around sun. Similarly, it would be contradictory to say both that Jeff knows the earth revolves around the sun and that it is not true that the earth revolves around the sun.

The appeal of the third condition—justification— can be seen by considering some education examples. If a high school student says he is sure (belief) that all squares are rectangles (true proposition) but struggles to give an adequate reason for this belief (“I can’t remember seeing a square that wasn’t a rectangle,” “I think they might be just two names for the same thing,” … ), we wouldn’t say that he knows this. He happens to believe it, and it’s true, but because he doesn’t have a good reason for believing it, he doesn’t really know. He’s just making a lucky guess. To know it, he would need to gain good reasons for believing it.

Gettier’s Critique and Responses

Philosophers have identified problems with the JTB analysis that center on the justification condition. In a brief article, Edmund Gettier generated examples to show that some justified true beliefs do not match the intuitions we have for what should be called knowledge. In the cases he poses, as in many other cases philosophers have devised and discussed in the subsequent decades, the problem comes about because the justification does not function to support the true belief. Instead, the justification contains a fatal flaw, and the truth of the proposition arises instead from some lucky accident, a case of “epistemic luck.”

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