The belief state of a rational agent may be viewed as consisting of sentences that are either beliefs, disbeliefs or neither (non-beliefs). When probabilities are used to model the belief state, beliefs hold a probability of 1, disbeliefs a probability of 0, and non-beliefs a probability between 0 and 1. Probabilistic belief contraction is an operation on the belief state that takes a belief as input and turns it into a non-belief whereas probabilistic belief revision takes a disbelief and turns it into a belief. Given a probabilistic belief state P , the contraction of P by an input a is denoted as Pa− and can be determined as the mixture of P and P ∗a, where P ∗a is the belief state that is a result of revising P by ¬a. The proportion of P and P ∗ that are used in the mixture is set by the mixing factor. Thus, the mixing factor has an important role to play in determining the contracted belief state Pa−.