Theory of justification is a part of epistemology that attempts to understand the justification of ideas that are "believed" or "known" to be true. Epistemologists are concerned with various epistemic features of belief, which include the ideas of justification, warrant, rationality, and probability. Of these four terms, the term that has been most widely used and discussed by the early 21st century is "warrant". Loosely speaking, justification is the reason why someone (properly) holds the belief, the explanation as to why the belief is a true one, or an account of how one knows what one knows.