When the occurrence of a or not-a happens under exactly the same circumstances given b, no causal relationship between individual b and individual a can be established.

Some philosophers have not found this argument very convincing. Faye (1994) has pointed to the following problematic issues. First, Mellor measures the probability of singular events (propensities) instead of the probability of certain kinds of events. Second, he does not differentiate between circumstances in which a \(B\) is followed by an \(A\) and those in which a \(B\) is not followed by an \(A\). The argument is valid only if it can be proved, and not be stipulated, that (1) and (3) happen