Spinoza’s view that there is just one substance is immensely attractive, but can we give any argument for it? How do we rule out many-substance views? We are accustomed to considering dual substance views such as Descartes’ dualism of mind and matter, but these don’t exhaust the field. We can formulate a fourfold substance metaphysics: Kantian things-in-themselves which are not spatial, perceptible matter which is spatial, finite minds such as our own, and the infinite mind of God. Arguments can be given to show that these are four fundamentally different kinds of substance; and of course we could reduce the number to three by omitting God or noumena. In principle there is no limit to the number of distinct substances there could be (we could add abstract substance if we so desired); we are not restricted to monism and dualism. We could stipulate a world in which there is a plurality of (types of) substance even if ours is not such a world. The question is what could count against the idea of a world of many substances.
What would count in favor of it is causal isolation. If two substances could not causally interact, that would indicate that they have fundamentally different natures, so different that any causal commerce between them is ruled out. This is the case for the abstract and the concrete: here causal interaction seems out of the question (the abstract entities aren’t even in space and don’t change). If our minds cannot interact with the mind of God in such a way as to change it, then that is a reason to suppose that the substance of God is not like our substance. Similarly, if mind and body cannot interact that would suggest that these are distinct substances: they don’t interact because they are not of such a nature that they could interact. Causal isolation is evidence of a diversity of fundamental nature. All physical things can in principle interact with each other, so they belong to a single category of substance; but if some could not, that would be a reason to suspect diversity of substance. If we came across a world that exhibited marked causal isolation between mind and body, or between our minds and God’s mind, that would indicate that we are in the presence of distinct substances.
The standard objection to Descartes’ dualism is that such different substances could not possibly interact. How could a substance whose essence is extension cause changes in a substance lacking extension? And how could extensionless thoughts bring about changes in extended bodies?  Yet mind and body do interact, so there can’t be a duality of substances. This suggests a criterion of identity for substances: substances are the same if and only if they can causally interact. If they can’t interact they are not the same, and if they can they are. Now the question is how causality actually operates in our world—does it connect everything with everything? It doesn’t connect the abstract to the concrete, so here we have deep ontological diversity; but it does connect mind and body (and it might connect noumena and phenomena or finite and infinite spirits). So we can argue as follows: since mind and body causally interact they must be of such a nature that they can so interact, but that requires a commonality of nature at a deep level. It can’t be that the whole nature of matter is extension (or gravity or electricity) while the whole nature of mind is lack of extension (or lack of gravity or electricity); there has to be some common ground at some level.  Thus these attributes must be aspects of an underlying substance that unifies what we call mind and matter—essentially Spinoza’s position. If we reject causal interaction between mind and body, we can insist on a duality of substance (as with Leibniz’s pre-established harmony); but if we allow it, then we face the question of what makes it possible. If causation runs right through the world in a single giant web of causal connection, then the world must be fundamentally unified, i.e. there is just one substance. And it does appear so to run.
This doesn’t mean that we can conceive the world as unified, except in a very abstract sense; our concepts and perceptual perspective may block us from appreciating its unified nature. But we can appreciate that causation runs through everything with no causal blocks (not counting the abstract), so we have a basis for supposing substance monism. If a subjective percept is caused by an objective stimulus, then cause and effect must share an underlying nature—the two must belong to the same “world” (similarly for noumena and phenomena, or our minds and God’s mind). Hume called causation the cement of the universe; well, it is a universal cement—a thread that sews everything together (to change the metaphor). Things cannot differ that much if they are in regular causal connection with each other—if interaction is natural to them. For causation must work intelligibly not magically (that’s why we balk at action at a distance): causes cannot bring about effects in things that share no properties with them. If A and B are radically different kinds of thing, they cannot causally communicate—which is why Descartes’ dualism has so much trouble with causal interaction. He has defined matter and mind in such a way that they cannot interact, since their entire essences are antithetical to each other. It would be different if, in addition to extension and thought, he had credited matter and mind with further properties that bring them together; then he could maintain that the interaction works by means of these further properties, whether known or unknown. The problem is that he has defined the two in such a way that their whole nature is distinct. Where Spinoza sees two modes of a single underlying world substance, Descartes sees an irreducible duality of substances: but then he has no account of causal interaction. He is trying to have it both ways. But granted causal interaction, a unity of substance is the indicated conclusion. Causation dissolves ontological distinction. A causally connected world is an ontologically unified world.
 Note that there is not supposed to be a problem about causation within each domain; in particular, it is not supposed that the immaterial mind is incapable of causal relations involving itself alone (thoughts causing other thoughts). The problem concerns causal relations across substance boundaries, in which cause and effect are of vastly different types.
 The cause must have an active power capable of bringing about the effect it does, and the effect must have a passive power which makes it capable of being brought about by that type of cause; both powers must have a basis in the categorical properties of cause and effect. We can’t just jam any two kinds of thing together and call them cause and effect: they have to suit each other in order to be joined as cause and effect.