Knowledge is more than belief. You can believe something false. You can believe something true by accident. Knowledge requires justification. The standard analysis, from Plato's Theaetetus, defines knowledge as justified true belief. You know something if it is true, you believe it, and you have good reasons for believing it. Edmund Gettier undermined that definition in 1963 with cases where someone has justified true belief but still does not know. In one case, Smith has evidence that Jones owns a Ford, and infers that either Jones owns a Ford or Brown is in Barcelona. Jones does not own a Ford, but Brown happens to be in Barcelona. Smith has justified true belief, but he does not know.

Philosophers have proposed many responses. Some add a fourth condition, such as no false lemmas or reliability. Others argue that knowledge is a primitive concept. Still others shift attention to the process by which beliefs are formed. Externalists hold that justification depends on facts about the knower's environment, not just on what the knower can access. Internalists hold the opposite. The debate continues, and Gettier cases remain a test for any theory of knowledge.

Types of knowledge

• Propositional. Knowledge that something is the case.
• Procedural. Knowledge how to do something.
• Acquaintance. Direct familiarity with something.
• Tacit. Knowledge that is difficult to articulate.

Knowledge is not the same as certainty. You can know something and still be open to revision.