 M2 is an anomalous model, in which the fluent ceases spontaneously. This model does not satisfy Condition (3), since M3 = &lt;I2,{I1, I2}&gt; also satisfies T; in particular, it satisfies the inertial axiom for f because it fails to satisfy Holds(f,s1). So, while M1 is a preferred model, M2 is not.