Epistemology (from the Greek, epistēmē, meaning 'knowledge', and logos, meaning 'logical discourse') is the branch of philosophy concerned with the theory of knowledge. Epistemology studies the nature of knowledge, justification, and the rationality of belief. Much of the debate in epistemology centres on four areas: the philosophical analysis of the nature of knowledge and how it relates to such concepts as truth, belief, and justification; various problems of skepticism; the sources and scope of knowledge and justified belief, and the criteria for knowledge and justification. Epistemology addresses such questions as 'What makes justified beliefs justified?', 'What does it mean to say that we know something?' and fundamentally 'How do we know that we know?'.