Following the tradition of permission as the sufficient condition for ideality, our paper applies the epistemic notion of ceteris paribus to measure the scale of normative sufficiency, in order to capture human reasoning about norms in natural language. We observe that ceteris paribus influences permission in two aspects, one is purely epistemic and the other normative. A sound and complete dynamic logic is developed accordingly, which can be seen as a generalization of Girard and Triplett's dynamic account. This formal theory is suitable to illustrate the deliberation about permission in Lewis' language game. Further, a trial application to the Hi-Lo games is shown.