  This solution comes at a price: it forces us to abandon the belief that pain is a single, natural kind. There is, however, a way to preserve this doctrine while pursuing a strategy that’s otherwise similar to the one just sketched. The essential idea is that “property” as we’ve used the term so far is ambiguous. A property could be what characterizes an object (event), or what unifies several objects as a “one across many”. Now suppose the characterizing properties are  tropes:  particularized properties, unique to each object. And suppose the  unifying properties are something else—call these  “types”. If the mental “properties” that  are causally relevant to behavior are tropes, and the mental  “properties” mentioned in the multiple realizability argument are types, there’s no reason to think that this argument rules out psychophysical property-identities in any way that leads to exclusion worries. The M-trope and the P-trope are one and the same trope falling under two types, mental and physical.  This proposal allows for a single type pain shared by diverse creatures; it’s just that this type is not the same sort of entity (a trope) that’s efficacious in the production of behavior (Heil 1992; Robb 1997; Heil and Robb 2003; what appears to be a similar view is defended by Macdonald and Macdonald 1986, 1995a; see also Whittle 2007.)