Hume's Combinatorial Modal Theory

David Hume is committed to the claim that the imagination can separate, mix, and recombine its objects at will. Given the close connection between what is imaginable and what is metaphysically possible for Hume, we can learn about the range of metaphysical possibility by investigating the limits of the Humean imagination. In this paper, I argue that Hume is committed to the view that for any external relation (spatiotemporal, causal, or identity-over-time), if that relation could be imagined among some objects, then it could hold among any objects which can be imagined in at least one relation of the same type. I apply this result to Hume's argument that causes must precede their effects, showing that this recombination principle of external relations licenses an inference which is otherwise difficult to justify on Hume's behalf. This provides some evidence that Hume was aware of the combinatorial implication of his modal theory, and appealed to it to support his arguments about causation