  This is one of the few points about the Frame Problem made by a philosopher  that raises a genuine difficulty for the formal solutions. But the difficulty is peripheral, since the example is not realistic. Recall that fluents are represented as first-order individuals. Although fluents are situation-dependent functions, an axiom of comprehension is certainly not assumed for fluents. In fact, it is generally supposed that the domain of fluents will be a very limited set of the totality of situation-dependent functions; typically, it will be a relatively small finite set of variables representing features of the domain considered to be important.  In particular cases these will be chosen in much the same way that a set of variables is chosen in statistical modeling.