 For several years Salmon thought that the combination of screening off and the common cause principle offered a solid basis on which causal explanation could be made to rest, but he later revised his position, after criticism from a number of authors including Bas van Fraassen (1977 and 1982). In order to cope with the difficulties besetting conjunctive forks Salmon formulated another type of fork, called interactive. Interactive forks depict causal interactions whose effects remain correlated even in the presence of the common cause; in other words the common cause does not screen off one effect from the other. The structure of interactive forks is analogous to that of conjunctive forks, with the difference that in this case the equality