---

Notice Mackie's distinction between necessity-one and necessity-two, in relation to proposition (k).

Neccesity-one: the distinguishing feature of causal as opposed to non-causal sequence

Neccesity-two: the warrant for an a priori inference between antecedent and consequent

Mackie argues:

But once we have drawn this distinction, we must concede that while Hume has a strong case for asserting k about necessity-two, he has no case at all for asserting it about necessity-one. (13)