Epistemology is the study of knowledge. It asks what knowledge is, how we get it, and how much we can have. The standard analysis, dating to Plato's Theaetetus, defines knowledge as justified true belief. You know something if you believe it, it is true, and you have good reasons for believing it. Edmund Gettier challenged that definition in 1963 with cases where someone has justified true belief but does not seem to know. The search for a better analysis continues.

Epistemologists also study sources of knowledge: perception, reason, memory, testimony, and introspection. They ask whether knowledge requires certainty or whether fallible justification is enough. They debate internalism, which holds that justification depends on what is accessible to the knower, and externalism, which holds that it depends on facts about the knower's environment. They investigate skepticism, the view that knowledge is impossible or very limited. They also study social epistemology, which examines how knowledge is produced and transmitted in communities, and formal epistemology, which uses probability and logic to model belief and evidence.

Central questions

• Definition. What is knowledge?
• Sources. Where does knowledge come from?
• Justification. What makes a belief rational?
• Skepticism. Can we know anything at all?

Epistemology matters beyond philosophy. It informs science, law, education, and any field where evidence and truth are at stake.