 Cartwright claims that causes raise the probabilities of their effects across all background  contexts,[4]  but many purported causes only raise the probabilities of their effects in some contexts. In the latter cases, causes interact with background factors in producing their effects. To give Cartwright’s own example (1979: 428), ingesting an acid poison generally causes death, except in contexts where one also ingests an alkali poison (in which case the two cancel one another out). The problem of such interactive causes for probabilistic accounts is that they threaten Cartwright’s picture on which the effect of probability-raising causes is “concealed” by a stronger negative cause which “dominates” them. These metaphors suggest that the probability-raising relationship between a cause and its effect reflects an intrinsic relationship between the variables that exists even when not manifested, an idea further developed in (Cartwright 1989). Interaction means that causes do not operate in a vacuum, but rather only in the presence of background factors (for further discussion, see Otte 1985; Eells 1986; Hardcastle 1991).