Some argue that mechanisms have to be regular in this factual sense (Andersen 2014a,b); i.e., repeated on many occasions (see Leuridan 2010). This view would seem to require a somewhat arbitrary cut-off point in degree of regularity between things that truly count as mechanisms and those that do not. Some mechanists (Bogen 2005; Glennan 2009) argue that there is no difficulty applying the term “mechanism” to one-off causal sequences, as when an historian speaks of the mechanism that gave rise to World War I. Other mechanists argue that the type-token distinction is too crude a dichotomy to capture the many levels of abstraction at which mechanism types and tokens might be characterized (Darden 1991).