 When one considers the initial example that more than 75% of all birds fly, one finds that this cannot be adequately captured in a model where the domain contains objects that are not birds. These objects should not matter to what one wishes to express, but the probability quantifiers, quantify over the whole domain. In order to restrict quantification one must add conditional probability operators \(Px (\phi | \psi) \geq q\) with the following semantics: