The sequence of events leading up to some final effect, where each member of the sequence causes its succeeding member to come about. While the notion applies to identifiable cases (the stumble causes the fall that causes the fracture that causes the visit to hospital…), it is not clear that causation in general can be analysed into linear chains of discrete events. Amongst classical problems is that of time: if the earlier event precedes the later, how can it be causally efficacious after it has ceased to exist? But if simultaneous, how do we find the chain stretched back in time?