   Suppose that the Pence and Ramsey proposal adequately deals with the counter-example problems that confronted earlier probabilistic propensity definitions of fitness. The question of whether this definition is adequate for trait fitness as well as for individual fitness and fitness differences remains. To begin with there is matter of whether the definition merely provides units in which to measure fitness the way that plainly operational definitions do, by contrast with definitions of dispositions in terms of their occurrent bases. The supervenience of fitness on a vast and heterogeneous base makes such a reductive definition impossible and as we have seen the ecological characterization in terms of “design problems” has its limits as well. Moreover, some have argued that the propensity definition is at most only a component of the meaning of trait fitness. As noted above, some philosophers (e.g., Ariew and Ernst (2007, 296)) argue that trait fitness is often a matter of population size and structure and conditions of genetic heritability, and these factors are “extrinsic to the causal properties of the individual” in its local environment. From this conclusion it seems an easy matter to infer that as it functions in the theory of natural selection, trait fitness may not be causal force at all. It may, after all, be a “book-keeping” notion, one which allows us to make predictive inferences without having any “biological reality”.