Here is an excellent paper on the distinction between coherence and constancy in Hume's Treatise 1.4.2, by Tim Black. (Hat-tip: OPP.) I have a particular interest in this subject; I delivered a paper on it at the international Hume Society meeting in 2003, which in part covered the same issue. I came to some conclusions that were similar, although I definitely didn't present it so neatly and well.
There are, however, a lot of things with which I disagree He says, for instance, that constant perceptions cannot cohere (footnote 34). I'd have to dig up the exact references, but I'm unconvinced by his reasoning for it; coherence can still be relevant in such cases because presumably one has had more than just perceptions of exactly the same thing. I also consider his reasoning for his 'radical proposal' simply wrong: the two statements he gives by Hume suggest the same thing from two different perspectives, not two different kinds of coherence-based beliefs in the external world. This is, I think, strongly suggested when we look at other places where Hume appeals to coherence. And I think he misunderstands what Hume means by "beyond the extent in which [the senses] really operate," which just means 'unperceived'. But he is certainly right about Price's interpretation of Hume, and the general approach is the one that's needed - the 'unified explanation approach' is simply wrong, despite its popularity.
What Black does not do, and needs to do, is to see how his interpretation of coherence in 1.4.2 fits with the use of similar principles in 1.2.4 on equality (Hume specifically refers back to this passage in 1.4.2). Any interpretation of the discussion of coherence in 1.4.2 must be consistent with what Hume says in 1.2.4 on how we get the idea of perfect equality. (This was essentially the topic of my paper - clarifying the 1.4.2 discussion in light of the 1.2.4 discussion. Were I to write it today, it would be rather different, since my view on some key matters has shifted; but the basic points are, I think, still right.) But it's great to see work actually being done to move away from the Pricean influence that has stunted much of the discussion of Hume's account of coherence-based belief in bodies that continue to exist unperceived.
