David Hume’s modern successors sometimes speak of a “mosaic” of particular facts or events. In my last post, I wrote:
The “Humean mosaic” is the information about local particular facts – properties at particular spacetime locations. I’ll come back to that next time.
I should have said “properties and relations” or maybe even “properties and relations held by objects.” (However, David Hume himself was very skeptical about objects, leading to first part of the quip by some of his critics, that Hume’s philosophy amounted to “No matter, never mind.”)
So, modern Humeans want to analyze natural laws as being convenient summaries of – and therefore secondary to – matters of properties and relations occurring at particular spacetime locations. In philosophy jargon, Humeans say laws are supervenient upon the local matters of fact. For examples of local facts: the ignition of the stove at 5:00 pm and the boiling of water at 5:05 pm. Or the travel of a particular water molecule away from its neighboring water molecules at 5:05:00.0001 pm. Etc.
But wait a second. Independent of other events in spacetime and independent of laws, what does it mean to say “water” or “boiling” or “travel”? Put aside for a moment (but not forever!) the question of how we know that something is water. What would it mean to say that’s water – what would water-ness be – if we take away any implications about what happened at the previous moment and what happens at the next moment? What if a “water” molecule need not break down salt, need not attract other water molecules with its electrical dipole, need not do any of the things water does? If a water molecule could start doing what an elephant does, and vice versa, what on earth distinguishes “water” from “elephant”? It’s not like objects each contain a tiny label written in Mandarin stating what kind of object they are. Nor do properties and relations bear labels. Nor do locations in spacetime.
For a water molecule (or whatever) to travel from A to B, there must be a lot in common along the spacetime path from A to B, enough to let us trace the “water molecule” fact along this path. It’s not like there is a label on the molecule saying (translated from the Mandarin) “I am water molecule #72.” Well, some philosophers might want to go near there, but definitely not a Humean who wants a lean, mean, science-friendly ontology.
Instead of making laws supervenient upon, i.e. secondary to, local properties and relations – instead of playing these metaphysical penis envy games – we need a package deal of laws and properties. The phrase “package deal” is Barry Loewer’s, and apt – it puts laws and properties on a par both metaphysically and epistemically.
No ghost, no machine, only human 
I’ve never read Hume directly (at least not to any great extent). But I’ve always interpreted what I read about his position to be, not that there is no necessity to the laws, but that we should be cautious in thinking we know what they are. All we have direct access to are the relations and correlations, not the laws themselves, which we can only infer. Future observations may force us to change our inferences, black swans, etc.
But I may be steelmanning him too much. It certainly seems like there has to be some necessity of some kind.
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I think you’re right that Hume wasn’t that concerned with metaphysics, he was concerned with epistemology. But that said, he had a very dismissive attitude toward anything we (supposedly) couldn’t know. “If we take in our hand any volume; of divinity or school metaphysics, for instance; let us ask, Does it contain any abstract reasoning concerning quantity or number? No. Does it contain any experimental reasoning concerning matter of fact and existence? No. Commit it then to the flames: for it can contain nothing but sophistry and illusion.”
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