 In another important early debate philosophers set out to solve a conceptual problem within biology itself. The concept of reproductive fitness is at the heart of evolutionary theory, but its status has always been problematic. It has proved surprisingly hard for biologists to avoid the criticism that, “[i]f we try to make laws of evolution in the strict sense we seem to reduce to tautologies. Thus suppose we say that even in Andromeda ‘the fittest will survive’ we say nothing, for ‘fittest’ has to be defined in terms of ‘survival’” (Smart 1959, 366). This came to be know as the “tautology problem”. Alexander Rosenberg and Mary B. Williams argued that fitness is an irreducible primitive which derives its meaning from its place in an axiomatic formulation of evolutionary theory (Rosenberg 1983; Sober 1984a; Williams and Rosenberg 1985). If correct, this would solve the tautology problem since axioms are often thought of as tautologous. In the 1970’s the new generation of philosophers of biology offered a different solution to the tautology problem in two steps. First, they began by arguing that fitness is a supervenient property of organisms: the fitness of each particular organism is necessarily dependent on some specific set of physical characteristics of the organism and its particular environment, but two organisms that have the same fitness may do so in virtue very different sets of physical characteristics (Rosenberg 1978). Second, they argued that this supervenient property is a propensity – which is a probabilistic disposition represented by a probability distribution over possible numbers of offspring (Mills and Beatty 1979). Although fitness is defined in terms of reproductive success, it is not a tautology that the fittest organisms have the most offspring, any more than it is a tautology that dice produce even numbers more often than they produce sixes. The propensities of fit organisms to survive and of dice to fall equally often on each side both allow us to make fallible predictions about what will happen, predictions that become more reliable as the size of the sample increases. It remains unclear, however, whether it is possible to specify a probability distribution or set of distributions that can play all the roles actually played by fitness in population biology (Beatty &amp; Finsen 1989, Sober 2001; Pence &amp; Ramsey 2013).