Epistemology (ᵻˌpɪstəˈmɒlədʒi; ) is the branch of philosophy concerned with knowledge, and is considered a major subfield of philosophy, along with other major subfields such as ethics, logic, and metaphysics. Epistemologists study the nature, origin, and scope of knowledge, epistemic justification, the rationality of belief, and various related issues. Debates in epistemology are generally clustered around four core areas: The philosophical analysis of the nature of knowledge and the conditions required for a belief to constitute knowledge, such as truth and justification Potential sources of knowledge and justified belief, such as perception, reason, memory, and testimony The structure of a body of knowledge or justified belief, including whether all justified beliefs must be derived from justified foundational beliefs or whether justification requires only a coherent set of beliefs Philosophical skepticism, which questions the possibility of knowledge, and related problems, such as whether skepticism poses a threat to our ordinary knowledge claims and whether it is possible to refute skeptical arguments In these debates and others, epistemology aims to answer questions such as "What do people know?", "What does it mean to say that people know something?", "What makes justified beliefs justified?", and "How do people know that they know?" Specialties in epistemology ask questions such as "How can people create formal models about issues related to knowledge?" (in formal epistemology), "What are the historical conditions of changes in different kinds of knowledge?" (in historical epistemology), "What are the methods, aims, and subject matter of epistemological inquiry?" (in metaepistemology), and "How do people know together?" (in social epistemology). The etymology of the word epistemology is derived from the ancient Greek epistēmē, meaning "knowledge, understanding, skill, scientific knowledge", and the English suffix -ology, meaning "the science or discipline of (what is indicated by the first element)".