I will discuss a question at the outer edge of human comprehension. Some metaphysical views are monist, some dualist, and some pluralist. Monist views include materialism and idealism; dualist views typically divide reality into the mental and the material; and pluralist views include a variety of disparate types of being. A standard form of pluralism would list matter, space, time, mind, value, and number—with possible subdivisions within these broad categories. The question I want to discuss concerns pluralism of this type—the view that reality divides into several distinct categories and is not unified by any overarching category (or pair of categories). Thus matter is not the same as space, time is a distinct dimension of reality, mind is separate from matter, value is not reducible to any of the aforementioned, and mathematics deals with its own sphere of existence. We might further allow that matter and mind allow for subdivisions and are not themselves homogeneous categories. The picture is that reality comes intrinsically divided up into several types of being with no unifying ontological structure. The components may be connected in various ways, causally and otherwise, but there is no unity at a basic level: reality is fundamentally a list.  Alternatively, reality is disjunctively defined: to be real is to be either matter or space or time or mind or value or mathematics. Reality is a mixed bag, a jumble, a heterogeneous collection. Hopes of unification are therefore misplaced.
This seems unsatisfying. Why should reality be thus divided? Why should the universe be so lacking in unity? It seems thrown together, a mere assemblage, with neither rhyme nor reason. Why would God construct a universe so irreducibly diverse? Why would the laws of nature allow for such a miscellany? Monism allows for unification, and dualism at least keeps the ontological number low, but pluralism accepts arbitrary amounts of variety, gleefully so. Pluralism accepts that the inventory of what there is could go on indefinitely as a matter of principle. It all seems so chaotic and contingent—as if the universe is just a disorganized pile. The universe is really a multiverse: we might as well speak of there being as many universes as there are components of the single one. Compare games like chess or tennis: if we just listed their component parts—white and black pieces of different shapes, a board, squares; racquets, balls, a net, a court—we would be faced with a mere random assembly. It is only when we place these items in the context of a unified game that a sense of the whole emerges. Then the pluralistic universe is like the components without the game: these elements are not part of anything with a unifying theme. But why a pluralistic universe and not a unitary universe? Could there be some unknown unifying reality behind the apparent diversity? The nearest we come to unity is via mathematics: matter, space, and time all submit to mathematical description, and perhaps the mind admits of some mathematical characterization too. But value seems not to be mathematically subsumable, and the mind is not wholly mathematical. Perhaps it would be different if God were behind everything; at least then we might discern a unifying purpose to the apparently fragmentary nature of the universe. But an atheistic perspective leaves reality in a state of radical disunity: the universe just happens to jam together quite disparate elements, like an overstuffed suitcase. That is, metaphysical pluralism presents us with an unintelligible jumble of heterogeneous parts. The parts are notoriously hard to connect together (e.g. the mind-body problem and the problem of mathematical knowledge), but there is the more basic worry that they exist at all in such variety. It offends a natural desire for simplicity, or at least orderliness. If God were designing a universe, why would he impose such chaotic diversity on it? Why the desire for the new and different? Or again: if there are other universes existing alongside this one, do they all display such disunity? Surely there are non-pluralistic possible worlds, so why is the actual world condemned to unexplained plurality? It may well be that pluralist metaphysics is logically inescapable for the actual world, but that still leaves the question of why pluralism should be true to begin with. Why must we have the material and the spatial and the temporal and the mental and the normative and the mathematical? The urge towards monism becomes understandable once we appreciate that pluralism is not intellectually satisfying as a final metaphysical stance. For it leaves us with an impression of jangling meaninglessness, obdurate incoherence, and queasy randomness—the world should not be so sundered and splintered! It is as if the universe lives several different independent lives. If universal panpsychism were true, we would have the result that the universe is made up of several completely different types of mind, each alien to the other (the material mind, the spatial mind, the temporal mind, etc.). Reality, under metaphysical pluralism, is composed of far too many realities, if I may put it so. And this is not a point about the types of reality making up Reality, but about the sheer number of them. What if pluralism said that reality as a whole is made up of 283 basic types of reality? Why is 6 the magic number?
Compare biological and astronomical reality. The biological world contains many types of animal—zoological pluralism is the norm—but this variety is underlain by a unifying theory, viz. the theory of evolution. So there is a fundamental unity to the variety we observe: the plurality isn’t just brute and inexplicable. Similarly, astronomy has discovered a plurality of galaxies where once we suppose unity, but this plurality is explained by the origin of the universe in the big bang (combined with the laws of nature)—it isn’t just a brute fact. But the plurality of the universe, as conceived by metaphysical pluralism, does seem like a brute fact (and is so intended by the metaphysical outlook in question). It seems distinctly arbitrary, surplus to requirements, and suspiciously de trop. It is not a plurality we can comfortably live with; it makes the universe seem casually put together, a mere congeries of qualitatively diverse elements. If we suppose (plausibly) that time is the original reality preceding all others, then the question time would have is why someone saw fit to keep introducing new realities in addition to time itself. It would be different if the additions were just modifications of time, but evidently they are completely different orders of being. Pleasing unity gave way to pointless disunity, gratuitous division. The universe became a mere bunch of sui generis elements. Metaphysical pluralism seems a bit too much like metaphysical anarchism. It may be true but it is hardly aesthetically pleasing or theoretically satisfying. Where was Occam’s razor when the universe needed it? Why the ontological multiplication for multiplication’s sake? And it is not as if the pieces fit snugly together, like pieces in a jigsaw; the pluralistic universe is by no means a harmonious universe. The architect of the universe was just making trouble for himself by insisting on irreducible plurality. The plural universe is a puzzling universe. Why Many instead of One? 
 I don’t of course mean that reality consists of words; the point is that reality is thought to be specified by a list of disparate elements each with its own nature.
 The question I am asking is not one I have seen addressed before, so it may seem odd or eccentric. I have tried to home in on the intuition in question, but it isn’t easy to do that. It might help if I say that the many types of reality envisaged by the pluralist look like so many types of stuff—and then the question is why so many types of stuff.
This entry raises an interesting question in an intriguing way. The list metaphor definitely deserves further elaboration. Not just for its obvious metaphoric quality, but also because it looks at the universe straight in the eyes, so to speak, and does not swipe the plurality of phenomena under the rug. Numbers, values, space, time, inanimate objects and sentient beings definitely appear as different as it gets.
I’m reasonably comfortable with the thought that ontology may be messy rather than neat.
What bothers me is that the mental and the physical seem to be intimately and causally related, but how can that be?
We must be physical but we can’t be.
I’m not so comfortable with that!