Humeans think they can imagine a brick coming into existence without any cause. This requires that not only they imagine a brick without imagining a cause, but that they imagine the brick and the absence of a cause. But the task of imagining absences as such is a difficult one. If I tell an ordinary person to imagine a completely empty room, the subject is likely to imagine an ordinary room, with walls but no furniture. But has the subject really imagined an empty room? Likely not. Most likely the imagined room is conceptualized in a way that implies that it has air in it. For instance, we could ask our subject what it would be like to sit in that empty room for eight hours, and our subject is unlikely to respond: "You'd be dead, since the room has nothing in it, and hence no oxygen either."
Could one with more directed effort imagine a room without any air in it? I am not at all sure of that. While we have the concept of vacuum as the absence of anything, it is not at all clear that we can imagine vacuum. Our language may itself be a giveaway of what we imagine when we imagine, as we say, a room filled with vacuum—indeed we are not really imagining an empty room. Moreover, most likely we are imagining the room as embedded in a universe like ours. But a room in a universe like ours will be pervaded with quantum vacuum as well as with electromagnetic and other fields, and perhaps even with spatial or spatiotemporal points. Whether these items count as things or not is controversial, of course, but at least it is far from clear that we've really imagined a truly empty room.
It is true that philosophers sometimes claim that they can imagine a world that consists only of, say, three billiard balls. But a claim to imagine that is surely open to question. First of all, the typical sighted person's imagination is visual. The balls are, almost surely, imagined visible. But if so, then it is an implicit part of what one is imagining that there are photons bouncing off the billiard balls. Furthermore, unless one takes care to specify—and I do not know how one exactly one specifies this in the imagination—that the balls obey laws very different from those of our world, there will constantly be occasional atoms coming off the edges of the billiard balls, and hence there will be a highly diffuse gas around the balls. Suppose all of this physics is taken care of by our careful imaginer. Still, have we really imagined a world containing only three billiard balls? What about the proper parts of the billiard balls—doesn't the world contain those? What about properties such as roundness, or at least tropes such as this ball's roundness? And aren't there, perhaps, relations between the balls? We see that unless one is a most determined nominalist, the content of the imagined world is going to be rather richer than we initially said. There are details implicit in the imagined situation which we have omitted.