  Dispositional concepts such as ‘magnetic’ have explanatory power even though they are defined in terms of causes and effects from which they are nevertheless distinct, and are realized by the molecular properties of matter on which they supervene. The fact that fitness is a probabilistic disposition makes for no special difficulty on this analysis. The charge of tautology against the theory thus rests on the mistaken demand that an explanatory variable must always be defined in terms distinct from its causes and effects. This is a demand shown to be unreasonable by the failed Logical Positivists’ attempts to give the empirical meanings of theoretical terms independent of their explanatory roles.