  There is however a much more serious issue facing the propensity definition of fitness: it has long been held to be difficult to pin down the specific mathematical expression of the probabilistic propensity that is supposed to constitute fitness altogether. The difficulty reflects features of natural selection that we must accommodate. Failure to deal with these difficulties leads inexorably to the conclusion that far from providing the theoretical meaning of fitness, the probabilistic propensity “definition” is a set of an indefinitely large number of operational measures of fitness; moreover, identifying these measures often turns on assuming as understood just what the theory of natural selection sets out to explain. And if this is so, the probabilistic propensity definition will not defuse the threat of triviality facing the theory of natural selection. We review here the research program of delivering a propensity definition of fitness that has sought to deal with these difficulties.