 Adopting [UP] will indeed allow us to go from (1) to (2). But before we can use it to establish that our causal inferences are determined by reason, we need to determine our basis for adopting it. [UP] is clearly not intuitive, nor is it demonstrable, as Hume has already pointed out, so only probable arguments could establish it. But to attempt to establish [UP] this way would be to try to establish probable arguments using probable arguments, which will eventually include [UP] itself.