 however one understands these conditional probabilities. The argument has prompted Fetzer and Nute (in Fetzer 1981) to offer a “probabilistic causal calculus” that looks quite different from Kolmogorov’s  calculus.[11]  But one could respond more conservatively, as Lyon (2014) points out. For example, Rényi’s axiomatization of primitive conditional probabilities does not have (∗) as a theorem, and thus propensities may conform to it despite Humphreys’ argument. Nonetheless, Lyon offers “a more general problem for the propensity interpretation. There are all sorts of pairs of events that have no propensity relations between them, and all three axiom systems—Kolmogorov’s, Popper’s, and Rényi’s—will sometimes force there to be conditional probabilities between them. This is not an argument that there is no alternative axiom system that propensity theorists can adopt, but it is an argument that the three main contenders are not viable” (124).