Latour claims that substances have no substrata. They are like threads that hold the pearls in a necklace. They are, I understand, like black boxes. Anything that is acts like an actant is good enough - if it can be treated like a pearl holder (a ground for a bundle of alliances or an assemblage), well, it is a pearl holder. There is no substrata, there are no dispositional properties, no potentialities, no vorhanden-heit.
And no causal powers. This is where Harman takes occasionalism to be close to Humean scepticism. Actualism is the common ground: something in act has to bridge the gap between causes and effects. The causal gap - like the existential gap - would disappear without a plurality of mediators: the world is animated because there is a plurality of agents eager to pay the price in alliances to get things done. For Hume, our habits alone do the job of bridging together the causal gap. The future is conceived by us to resemble the past. For Latour, our networks work in the present and the past to bring about a future - create lab-like conditions. Surely, something in the assemblage can break it apart and the sun can fail to rise tomorrow. It is a contingent matter, but this is not the end of the story (as there is much more to be told for Hume also). But the version of occasionalism espoused by Latour seems to imply that there ought to be always a mediating agent responsible for a causal link or the absence of one. If the sun fails to rise tomorrow, something in the network went rotten. This is, as it is expected, much more than what Hume would be prepared to say.
What bridges the gap? Other agents in a network that host substances like black boxes. Anything - what ever it is - that keeps, says, the pool table smooth. Now, there are assemblages of black boxes that seem like objects - they would be my cubist objects. Like the microbe that assembles the beer-brewer box, the milk-rotting box etc. (Cubist) objects need to be brought about, they don't hang out on their own. I thought they are not vorhanden but something like nachhanden. In any case, (cubist) objects are at most a special case of asseblage, of networks brought about.