 begins by noting that the facts expressed by  (1)  and  (2)  may not be “constantly conjoined”, but there’s at least a very strong correlation between the two: the great majority of cases of (1) are cases of (2), and the great majority of cases of (2) are cases of (1). More than that, given our knowledge of the world, it’s plausible to assume that there’s a causal relation between type-(1) facts and type-(2) facts: in general, type-(1) facts bring about or cause type-(2) facts. A universal causal law of the form, All M-type facts bring about or cause N-type facts, wouldn’t be quite correct in this case, but neither would it be in the case of throwing rocks and breaking windows. The causal generalization is sound enough in both, however.