The branch of philosophy dealing with knowledge is epistemology (from Greek, epistm 'knowledge' and -logy). Epistemologists research the nature, origins, and scope of knowledge, as well as epistemic justification, belief rationality, and other related topics. Epistemology, along with other main subfields such as ethics, logic, and metaphysics, is considered an essential subfield of philosophy.

In epistemology, debates usually revolve around four main topics:

  1. A philosophical examination of the nature of knowledge and the conditions that must be met in order for a belief to be considered knowledge, such as truth and justification.
  2. Perception, reason, memory, and witness are all potential sources of knowledge and justified belief.
  3. The organization of a body of knowledge or justified belief, including whether all justified views must be drawn from justified fundamental beliefs or whether justification requires merely a collection of beliefs that are cohesive.
  4. Philosophical skepticism, which examines the possibility of knowledge, and related issues, such as whether skepticism threatens our everyday knowledge claims and if skeptical arguments can be refuted.

Epistemology seeks to address questions such "What do we know?" "What does it mean to state that we know something?" "What makes justified beliefs reasonable?" and "How do we know that we know?" in these and other discussions.