Belief is the psychological state in which an individual holds a proposition or premise to be true.

Belief, knowledge and epistemology

The relationship between belief and knowledge is subtle. Believers in a claim typically say that they know that claim. For instance, those who believe that the Sun is a god will report that they know that the Sun is a god. However, the terms belief and knowledge are used differently by philosophers.

Epistemology is the philosophical study of knowledge and belief. A primary problem for epistemology is exactly what is needed in order for us to have knowledge. In a notion derived from Plato's dialogue Theaetetus, philosophy has traditionally defined knowledge as justified true belief. The relationship between belief and knowledge is that a belief is knowledge if the belief is true, and if the believer has a justification (reasonable and necessarily plausible assertions/evidence/guidance) for believing it is true.

A false belief is not considered to be knowledge, even if it is sincere. A sincere believer in the flat earth theory does not know that the Earth is flat. Similarly, a truth that nobody believes is not knowledge, because in order to be knowledge, there must be some person who knows it.

Later epistemologists have questioned the "justified true belief" definition, and some philosophers have questioned whether "belief" is a useful notion at all.

Belief as a psychological theory

SectOR: date=October 2007

Mainstream psychology and related disciplines have traditionally treated belief as if it were the simplest form of mental representation and therefore one of the building blocks of conscious thought. Philosophers have tended to be more rigorous in their analysis and much of the work examining the viability of the belief concept stems from philosophical analysis.

The concept of belief presumes a subject (the believer) and an object of belief (the proposition). So like other propositional attitudes, belief implies the existence of mental states and intentionality, both of which are hotly debated topics in the philosophy of mind and whose foundations and relation to brain states are still controversial.

Beliefs are sometimes divided into core beliefs (those which you may be actively thinking about) and dispositional beliefs (those which you may ascribe to but have never previously thought about). For example, if asked 'do you believe tigers wear pink pajamas ?' a person might answer that they do not, despite the fact they may never have thought about this situation before.

That a belief is a mental state has been seen, by some, as contentious. While some philosophers have argued that beliefs are represented in the mind as sentence-like constructs others have gone as far as arguing that there is no consistent or coherent mental representation that underlies our common use of the belief concept and is therefore obsolete and should be rejected.