 In the sciences, the idea that probabilities express physical states of affairs, often called chances or stochastic processes, is most prominent. They are relative frequencies in series of events or, alternatively, they are tendencies or propensities in the systems that realize those events. More precisely, the probability attached to the property of an event type can be understood as the frequency or tendency with which that property manifests in a series of events of that type. For instance, the probability of a coin landing heads is a half exactly when in a series of similar coin tosses, the coin lands heads half the time. Or alternatively, the probability is half if there is an even tendency towards both possible outcomes in the setup of the coin tossing. The mathematician Venn (1888) and scientists like Quetelet and Maxwell (cf. von Plato 1994) are early proponents of this way of viewing probability. Philosophical theories of propensities were first coined by Peirce (1910), and developed by Popper (1959), Mellor (1971), Bigelow (1977), and Giere (1976); see Handfield (2012) for a recent overview. A rigourous theory of probability as frequency was first devised by von Mises (1981), also defended by Reichenbach  (1938) and beautifully expounded in van Lambalgen (1987).