Propensities are invoked to explain why repeating a certain kind of experiment will generate a given outcome type at a persistent rate.
Frequentists are unable to take this approach, since relative frequencies do not exist for single tosses of a coin, but only for large ensembles or collectives.
[2][3][4][5] A later propensity theory was proposed[6] by philosopher Karl Popper, who had only slight acquaintance with the writings of Charles S. Peirce, however.
To say that a set of generating conditions G has propensity p of producing the outcome E means that those exact conditions, if repeated indefinitely, would produce an outcome sequence in which E occurred with limiting relative frequency p. Thus the propensity p for E to occur depends upon G:
In other words, non-trivial propensities (those that differ from 0 and 1) imply something less than determinism and yet still causal dependence on the generating conditions.
[11] They show that the causal nature of the condition in propensity conflicts with an axiom needed for Bayes' theorem.