 Most of these systems are not based on unary probabilities \(P(\phi)\), but rather on conditional probabilities \(P(\phi,\psi)\). The conditional probability \(P(\phi,\psi)\) is taken as primitive (rather than being defined as \(P(\phi\wedge\psi)/P(\psi)\), as is usually done) to avoid problems when \(P(\psi)=0\). Goosens (1979) provides an overview of various axiomatizations of probability theory in terms of such primitive notions of conditional probability.