 Unlike conjunctive forks, causal processes and interactions cannot be defined in purely statistical terms. To stress this crucial difference, Salmon referred to statistical causality in terms of screening off and conjunctive forks, and aleatory causality in terms of processes and interactive forks. Aleatory causality “places primary emphasis on the mechanisms of causality” (Salmon 1990c [1998: 207]) and informs us about how phenomena fit within the mechanisms responsible for their occurrence. Statistical regularities are all that is needed by statistical causality, whereas aleatory causality requires more, namely the possibility to speak of tendencies that are both causal and probabilistic. Salmon thought that propensities, taken as causal probabilistic tendencies  (§ 2.3),  prove fruitful in that connection, and held that: