 Prior et al. point out that their argument assumes that a disposition cannot be an overdetermining cause of its manifestation, more generally, that no event can have two ‘operative’ sufficient conditions (see also Jackson 1998, p. 202). Since, ex hypothesi, it is the causal basis for the disposition that is the operative sufficient condition for its manifestation, the disposition itself, even though its possession is sufficient in the circumstances for the manifestation, is an inoperative sufficient condition, and so is not a cause. Schaffer (2003), however, argues that cases of overdetermining causes in Prior et al.’s sense are not just possible but, as a matter of fact, routine. In his view, they must be modeled after ubiquitous cases of quantitative overdetermination where ‘the cause is decomposable into distinct and independently sufficient parts’: when one big round rock hits a window, it has two hemispheres that quantitatively overdetermine the breaking of the window. If Schaffer is right, then Prior et al.’s argument for the causal inefficacy of dispositions is defused. (See Rives 2005 for a different formulation of the argument that doesn’t assume that a disposition cannot be an overdetermining cause of its manifestation.)