 But there is an objection, most recently expressed by George Bealer (1997; see also Hill 1993), that, on this model an introspective belief can only be defined in one of two unsatisfactory ways: either as a belief produced by a (second-order) functional state specified (in part) by its tendency to produce that very type of belief — which would be circular — or as a belief about the first-order realization of the functional state, rather than that state itself. Functionalists have suggested, however (Shoemaker 2001, McCullagh 2000, Tooley 2001), that there is a way of understanding the conditions under which beliefs can be caused by, and thus be about, one's second-order functional states that permits mental states and introspective beliefs about them to be non-circularly defined (but see Bealer 2001, for a skeptical response). A full treatment of this objection involves the more general question of whether second-order properties can have causal efficacy, and is thus beyond the scope of this discussion (see section 5.2 and Mental Causation entry). But even if this objection can ultimately be deflected, it suggests that special attention must be paid to the functional characterizations of “self-directed” mental states.