 A probabilistic causal model also includes a probability measure P. P is defined over propositions of the form \(X = x\), where X is a variable in \(\bV\) and x is a value in the range of X. P is also defined over conjunctions, disjunctions, and negations of such propositions. It follows that conditional probabilities over such propositions will be well-defined whenever the event conditioned on has positive probability. P is usually understood to represent some kind of objective probability.