 This is best understood when put into action. Take the case with Tweety the penguin. We have \(\Delta' = \{ p \rightarrow \pi_1, b \rightarrow \pi_2\}\), \(\Gamma' = \{ p \supset b, p \wedge \pi_1 \supset \neg f, b \wedge \pi_2 \supset f\}\), and \(\Lambda = \{p\}\). Let \(\Pi = \{\pi_2\}\). Then \(\Pi, \Gamma', p \models \neg \pi_1\) and thus for &lt; to be admissible, \(\pi_2 &lt; \pi_1\). We have two types of models of \(\Gamma' \cup \Lambda\): models \(M_1\) for which \(M_1 \models \pi_1\) and therefore \(M_1 \models \neg f\) and models \(M_2\) for which \(M_2 \models \pi_2\) and thus \(M_2 \models f\). Note that \(M_1 \lessdot M_2\) since for the only assumption in \(Ab(M_1)\) —namely \(\pi_2\)— there is \(\pi_1 \in Ab(M_2) \setminus Ab(M_1)\) and \(\pi_2 &lt; \pi_1\). 