If I ask myself what are the most difficult topics in metaphysics, I find that the following three stand out: necessity, causation, and existence. The questions couldn’t be simpler to state: What is the nature of necessity? What is the nature of causation? What is the nature of existence? Yet they seem impossible to answer; we draw a blank. The intellect flounders. We feel that all we can do is repeat the words in italics. As we know, eager philosophers have not left it there—proposed answers abound. Some declare the questions to be illegitimate or meaningless; some deny that there is anything real to fret about; some try to provide actual theories, analyses, and explanations. My concern is more limited: to point out a structural similarity between three theories that have seemed to many people to be promising, even correct. I don’t think the theories work, but there is an instructive pattern in their not working.
Theory 1: Necessity is truth in all possible worlds (or the obtaining of a fact in all possible worlds). Theory 2: Causation is constant conjunction (the regularity theory). Theory 3: Existence is the instantiation of properties (or concepts or predicates). The pattern in the theories is clear: what looked like a singular fact involving an individual gets analyzed as a general fact by introducing a new range of entities. For the number 2 to be necessarily even is for 2 to be even in all worlds; for a to cause b is for a and b to be conjoined in all instances;  for Socrates to exist is forhis properties to be instantiated. In each case an apparently singular fact is represented as a collection of facts: facts in possible worlds, facts of regular succession, and facts of property instantiation. The idea is that the original facts emerge from the conjunction of these more basic facts: necessity emerges from truth in all worlds; particular causation emerges from general regularity; individual existence emerges from property instantiation.  The One comes from the Many. You add up the many facts and the one fact falls out as a consequence: you add up the possible worlds and you get necessity; you add up the conjoined instances and you get causation; you add up the property instantiations and you get existence. You can’t get the original fact from any one of the particular facts you have assembled, but you can get it from the totality of them. Necessity is a set of worlds; causation is a set of instances; existence is a set of instantiated properties. We expand our ontology and wield a universal quantifier, and the problematic fact pops out. We reduce our difficult concepts to concepts less difficult (allegedly) by means of this pattern of analysis. What looked like a confined and local fact turns out to consist of a wide-ranging totality of facts.
The intuitive objection to this procedure is twofold. First, the original facts don’t seem to be so distributed, so general, so encompassing: they seem to concern particular entities and nothing beyond them. Now we have to reckon with these new entities if we are to believe in the facts we started out with. But second, the new facts look more like consequences of the original facts than constitutive of them. Why is the number 2 even in all possible worlds? Because 2 is necessarily even—that is the reason 2 is even in all possible worlds. Why are a and bconstantly conjoined? Because a is the cause of b—that is the reason for the constant conjunction. Why are Socrates’ properties instantiated? Because Socrates exists—that is the reason his properties are instantiated. This is the correct logical order of explanation, not the claimed dependence of the original facts on the introduced facts. It is not that God first made the possible worlds, the regularities, and the instantiations, and then necessity, causation, and existence fell out as consequences. Rather, the latter facts are basic and the former derivative. The ground of the general facts is the singular fact they purport to analyze. That singular fact remains as inscrutable as ever. 
 There are detailed objections to the theories in question, which I have not discussed here; my point has been simply to note an abstract similarity in theories of three seemingly unrelated topics. Each theory involves an attempt to reduce the particular to the general, and each arguably puts the cart before the horse.