The evasive plausibility

A frequent practical problem is to assess the probability of a potential single event (X). The event probability P(X) is usually conditional to some assumption (A) and is then written P(X / A). Whether the assumption is valid or not is an unknown fact, but its validity may be assigned a weight psi(A) indicating the observer's belief in its validity. The weighted quantity would be the unconditional probability of X if the weighting factor were a true probability. However, it is not a stochastic quantity even though in Bayesian statistics it would be treated as a probability. It follows that the weighted quantity, i.e. psi (A) x P(X / A), is not an unconditional probability in the usual sense. It is suggested that it be given a special name, for example 'plausibility'.