The AGM Theory for Belief Revision is a framework developed by Carlos Alchourrón, Peter Gärdenfors, and David Makinson in the late 1980s that provides a systematic approach to modifying one's beliefs in the presence of new information. It stands for "Alchourrón, Gärdenfors, and Makinson" - the surnames of its originators.
According to the AGM Theory, belief revision is the process of modifying an agent's existing beliefs in light of new knowledge or evidence. It consists of three fundamental operations: belief contraction, belief expansion, and belief revision.
Belief contraction entails removing certain beliefs from the agent's belief set due to inconsistency or contradictions with new information received. This operation ensures that the revised set of beliefs remains logically coherent and avoids any contradictions.
Belief expansion, on the other hand, involves adding new beliefs to an agent's existing set of beliefs to accommodate the information received. These new beliefs should be logically consistent with the existing beliefs.
Belief revision is the most general operation, combining both contraction and expansion. It involves revising an agent's belief set by removing some beliefs and adding new ones to reflect the most reasonable response to the new information.
The AGM Theory provides a set of axioms and postulates to ensure that the belief revision process is rational and coherent. It aims to capture the essence of how rational agents should modify their beliefs when confronted with new evidence, allowing for consistency and reasonableness in the face of changing information.
