 Replacement aims both at the removal of a sentence \(p\) and the addition of a sentence \(q\). This amounts to two success conditions, \(p \not\in K[p/q]\) and \(q \in K[p/q]\). However, these two conditions cannot both be satisfied without exception. First, just as for belief contraction, an exception must be made from the first of them in the case when \(p\) is a tautology and so cannot be removed. Secondly, the two conditions are not compatible in cases when \(q\) logically implies \(p\). This can be dealt with by extending the exception that is provided for in the Success postulate for contraction from cases when \(p \in \Cn(\varnothing)\) to cases when \(p \in \Cn(\{q\})\). This gives rise to the following two success conditions: