 AGM theory studies three different types of belief change. Contraction occurs when the belief state \(\bB\) is replaced by \(\bB\div \alpha\), a logically closed subset of \( \bB\) no longer containing \(\alpha\). Expansion occurs when the belief state \( \bB\) is replaced with \(\bB+\alpha = \Cn(\bB\cup \{\alpha \})\), the result of simply adding \(\alpha\) to \(\bB\) and closing under logical consequence. Revision occurs when the belief state \(\bB\) is replaced by the belief state \(\bB*\alpha\), the result of “minimally mutilating” \(\bB\) in order to accommodate \(\alpha\).