 One significant limitation of the belief-revision approach is that there is no representation in the object-language of a defeasible or default rule or conditional (that is, of a conditional of the form If p, then normally q or That p would be a prima facie reason for accepting that q). In fact, Gärdenfors (Gärdenfors 1978; Gärdenfors 1986) proved that no conditional satisfying the Ramsey test can be added to the AGM system without trivializing the revision  relation.[1]  (A conditional \(\Rightarrow\) satisfies the Ramsey test just in case, for every epistemic state \(K, K\) includes \((A \Rightarrow B)\) iff \(K * A\) includes \(B\).)