Chapter 3 explored the possibilities of explaining the asymmetry of causation in terms of the relations between causation and time. Chapter 5 explored the possibilities of generating a theory of causal asymmetry from the connections between causation and agency. This chapter explores another attractive set of connections -those between causation and counterfactuals. When one asserts that one event a causes another distinct event b, then it seems that one is committed to the counterfactual: “If a had not occurred, then b would not have occurred either.” Hume himself wrote “ … we may define a cause to be an object, followed by another, and where all the objects similar to the first, are followed by objects similar to the second. Or in other words, where if the first object had not been, the second had never existed’ (Inquiry p. 51). The “other words” here are indeed “other words.” Hume seems mid-definition to leap from a regularity to a counterfactual theory of causation. This chapter explores the possibility of constructing a counterfactual theory of causation.

Lewis's Theory

According to David Lewis, if a and b are distinct events that actually occur, then b causally depends on a if and only if, if a were not to occur, then b would not occur either (1973a). If the match had not been struck, then it would not have lit. In Lewis's usage (in contrast to mine), “causal dependence” is a relation among token events, which is sufficient for causation. It is not the same thing as causation, because in cases of preemption it is not transitive.