 The best known and most thoroughly elaborated counterfactual theory of causation is David Lewis’s theory in his (1973b). Lewis’s theory was refined and extended in articles subsequently collected in his (1986a). In response to doubts about the theory’s treatment of preemption, Lewis subsequently proposed a fairly radical revision of the theory (2000/2004a). In this section we shall confine our attention to the original 1973 theory, deferring the later changes he proposed for consideration below.