   The difficulties that face both the subjective probability view of fitness and the probabilistic propensity definitions of fitness are serious enough to make the notion of “ecological fitness” worth revisiting. Recall that on this view ‘\(a\) is fitter than \(b\) in \(E\)’ is defined as ‘\(a\)’s traits result in its solving the design problems set by \(E\) more fully than \(b\)’s traits.’ The terms in which this definition is couched are certainly in as much need of clarification as is ‘fitness’. It is cold philosophical comfort to point out that definitions have to stop somewhere, that the definition of a theoretical term must be distinct from the operational measure of the property it names, and that testability is not a matter of theory meeting data one proposition at a time. In consequence none of these considerations have convinced philosophers of biology to accept the design problem definition, and to give up the project of defining fitness by reference to its effects in reproduction. Nevertheless, there does appear to be important biological work that the ecological fitness concept can do, and which no definition of fitness in terms of differential reproductive rates—actual, expected, or dispositional—can do.