  Consider one way this might work. Behavioral properties, just like mental properties, appear to be multiply realizable. For example, there is more than one way to hail a cab, many different physical realizations of this kind of behavior. Now suppose a belief causes you to hail a cab. In accordance with Completeness, some physical property P of the belief is sufficient for your behavior. But strictly speaking, P is relevant only to the particular way in which you hailed the cab, the particular physical realization of your hailing. What, then, is responsible for your behavior’s higher-level property of simply being a cab-hailing? It’s natural to suppose that it’s a  higher-level property of your belief, namely, some mental property,  such as the belief’s representational content. (For proposals along  these lines, see Yablo 1992; Thomasson 1998; Marras 1998; Crisp and  Warfield 2001; Gibbons 2006; Schlosser 2009; see also §§7.3–4.)