  Finally, there are those who question the assumption that factorizability is a locality condition (Fine 1981, 1986, pp. 59-60, 1989b, Cartwright 1989, chaps. 3 and 6, Chang and Cartwright 1993). Accordingly, they deny that non-factorizability implies non-locality. The main thrust of this line of reasoning is that the principle of the common cause is not generally valid. Some, notably Cartwright (1989) and Chang and Cartwright (1993), challenge the assumption that common causes always screen off the correlation between their effects, and accordingly they question the idea that non-factorizability implies non-locality. Others, notably Fine, deny that correlations must have causal explanation. 