Theory of justification is a part of epistemology that attempts to understand the justification of propositions and beliefs. Epistemologists are concerned with various epistemic features of belief, which include the ideas of justification, warrant, rationality, and probability. Loosely speaking, justification is the reason that someone (properly) holds a belief.
When a claim is in doubt, justification can be used to support the claim and reduce or remove the doubt. Justification can use empiricism (the evidence of the senses), authoritative testimony (the appeal to criteria and authority), or logical deduction.
Justification focuses on beliefs. This is in part because of the influence of the definition of knowledge as "justified true belief" often associated with a theory discussed near the end of the Socratic dialogue Theaetetus. More generally, theories of justification focus on the justification of statements or propositions.
The subject of justification has played a major role in the value of knowledge as "justified true belief". Some contemporary epistemologists, such as Jonathan Kvanvig assert that justification isn't necessary in getting to the truth and avoiding errors. Kvanvig attempts to show that knowledge is no more valuable than true belief, and in the process dismissed the necessity of justification due to justification not being connected to the truth.
Justification is the reason why someone properly holds a belief, the explanation as to why the belief is a true one, or an account of how one knows what one knows. In much the same way arguments and explanations may be confused with each other, as may explanations and justifications. Statements that are justifications of some action take the form of arguments. For example, attempts to justify a theft usually explain the motives (e.g., to feed a starving family).
It is important to be aware when an explanation is not a justification. A criminal profiler may offer an explanation of a suspect's behavior (e.g.; the person lost his or her job, the person got evicted, etc.), and such statements may help us understand why the person committed the crime. An uncritical listener may believe the speaker is trying to gain sympathy for the person and his or her actions, but it does not follow that a person proposing an explanation has any sympathy for the views or actions being explained. This is an important distinction because we need to be able to understand and explain terrible events and behavior in attempting to discourage it.