Causation is one of those philosophical topics that drive you up the wall. As soon as you start to think about it you draw a complete blank. As Hume observes: “There are no ideas, which occur in metaphysics, more obscure or uncertain, than those of power, force, energy, or necessary connexion” (Enquiry, p.45).  The cement of the universe is so much muddy water. Of course philosophers have done everything in their power to hide this fact from themselves, even going so far as to try to reduce causality to mere regularity. Hume’s own view was that causation is real but incomprehensible (by us). It is neither an affair of the senses nor of reason: we have no sensory impression of necessary connection (which is definitive of causation) but neither is causation grasped by reason (like logic, arithmetic, and geometry). It is a real relation between things but it is not revealed by perceiving them or by merely thinking about them. It fits neither empiricism nor rationalism. It sits uncomfortably between the two, awkwardly and inscrutably. No matter how much you gaze at an object or reflect on it you will never discover causation (as it exists in that object). But perception and reason are our only faculties of knowledge, so the mind draws a blank on the nature of causation. Yet we constantly refer to it, rely on it, and assume its reality. Evidently we can know that it obtains, relying on the observation of regularities of nature, but we can’t fathom its inner nature—or even fathom our lack of fathoming.
The problem concerns not just particular causal relations but also the notions of law and power (disposition, capacity, potential). Objects fall under causal laws and have causal powers: this is why they have the effects they have. But laws and powers are at least as inscrutable as causal relations between particulars; this is an interconnected knot of problems. Perhaps the notion of power concentrates the problem most acutely (as Hume intimates): how are the potential effects of a cause contained in it? Are the effects somehow already present in the cause? Does the cause “refer” to the effects? Are there shadows or signs of the effects lurking silently in the cause? But you can’t discern anything like this if you examine the cause, even going down to its elementary constituents. When a moving object imparts motion to another object by collision is the other object’s motion somehow prefigured in the moving object? It had the potential of creating that effect, so doesn’t it already contain it in someway? What is potential? It’s a bit like the way the meaning of a word “contains” its uses: they are implicit in it, packed into it—but what does this way of speaking amount to?  The problem of causation is how an object can contain what it does not contain. If we think of a cause as a conscious being for a moment, it is as if it knows what effects it can bring about, but only unconsciously: it doesn’t have these effects before its consciousness, but it is subliminally aware of them—they are implicitly known not explicitly known. But that looks like a dodge: they aren’t anticipated in any way that we can discern—the mind of a cause is blank about its future effects. Yet it has the power to bring precisely these effects about, and this power is internal to it, so… Thus the waters fill with mud.
What can we say positively about causation? The logic, semantics, and conceptual analysis of “cause” are not so baffling. Thus “x caused y” expresses a relation that is irreflexive, asymmetric, and transitive: nothing can cause itself, effects can’t cause causes, and the effects of effects are caused by the initial cause. Semantically, it is plausible to suggest that “cause” generates a transparent context and expresses a relation between events (though this view is not without its critics). The concept may also be analyzable in terms of counterfactuals or other necessary and sufficient conditions. So it is not that we can say nothing about the word and what it means—and much philosophical energy has been expended on these worthy tasks. But they don’t touch the underlying metaphysical and epistemological questions, the ones so memorably raised by Hume. What exactly is causation, and how do we know about it? Specifically, what is it to have a causal power, and how can we know causal powers? There are suggestions—there are always suggestions. One suggestion is that a causal power is identical to a structural property of the object, as it might be molecular structure. But this just pushes the question back: how is the power present in the structure? Isn’t it as invisible as ever? Nor can it be excogitated by pure reason. It can’t be seen and it can’t be deduced—it flouts both empiricist and rationalist epistemology. It isn’t a posteriori and it isn’t a priori. It is a peculiar kind of fact, being neither perceptible (even by extended perception: microscopes etc.) nor rationally apprehended. As Hume would say, it is neither a “matter of fact” nor a “relation of ideas”; it hovers ambiguously between the two. No wonder there has been a marked tendency towards elimination: causation must either be reduced to facts less problematic (regularities, dispositions to project) or eliminated outright. To accept it as it is runs into insurmountable metaphysical and epistemological difficulties. Indeed, it threatens to bring down the most fundamental structures of philosophical thought.
So why not just bring them down? Because we have nothing to put in their place, that’s why. It is not as if we have some other way to think about causation that we can substitute for the old inadequate dichotomies: the waters are thick with mud and our vision fails us. It is really a horrible problem. Best not even to go near it; just leave it alone to fester. But maybe we can articulate the problem better, gain a better sense of its dimensions and density. Can we at least pinpoint why it is so difficult? It is in some ways worse than the problem of consciousness, because in that case at least we know what we are talking about—we don’t just refer to consciousness, we experience it. But we don’t experience causation (as opposed to its symptoms), despite our readiness to refer to it. We refer to a we-know-not-what. Appealing to mental causation won’t help, despite our immediate acquaintance with the mental phenomena between which causation holds: for mental causation is as opaque as physical causation (as Hume noted). We can say that physical causation is no less active than mental causation; the will is not somehow a livelier form of causation. Nor is causation by physical contact more transparent than causation-at-a-distance, since its operation is as obscure there as it is in the remote case. In this respect old-style mechanism offers an illusory paradigm of transparency (this was Hume’s central insight, in effect): it isn’t that causation by contact is quite clearly grasped while causation-at-a-distance must be deemed “occult”. Neither is really intelligible to us, not when you get right down to it. Hume’s billiard balls hit each other, unlike orbiting planets, but their causal powers are no more evident to sense or reason than gravity. For some people this was taken as a reason to eliminate causation altogether from physics, and one can appreciate the motivation.
Can we be more constructive? I think we can say two positive things, though nervously. The first is that nature must be more tightly interlinked than we tend to suppose going by the appearances: causation connects things because any effect of a cause must be somehow written into the cause (though not in a way we clearly conceive). The colliding billiard balls don’t appear to sense perception as having any intelligible connection; nor can human reason discern any such connection: but they must somehow be intelligibly linked. The laws of nature essentially relate separate things, because causal powers are essentially powers to bring about certain specific effects: an object x has the power to make an object y have a property P. There is thus more “holism” at work in nature than is apparent to our epistemic faculties. We could introduce the idea of the “causal boundary” of an object to signify the class of objects that fall within its causal reach—for instance, the class of solids a given liquid can dissolve. This class falls within its causal boundary but not its spatial boundary. Then nature will be said to consist of the totality of such causal boundaries—these are the true units of nature.
The second thing we can say is that whatever causal powers are they must be very different from their manifestations in observable phenomena. This is because the manifestations never add up to a causal power, as it exists in objects. It can’t be mere regularity and it can’t be a “categorical base” (e.g. molecular structure)—these are not what the power is or else we would know what it is. Powers must be as different from their manifestations as mental states are from behavior—perhaps more so. Potentiality must be different from actuality; yet the two must be intimately related. I can’t tell you how potentiality differs from actuality because of its obscurity, but it evidently does differ, dramatically so. (Or else actuality is merely the way potentiality looks to our senses and doesn’t go deep ontologically speaking. )
Would other things become clearer if we had a better grip on causation? Anything in which causation directly figures would be—laws of nature, the origin of the universe, the operation of fundamental particles. But it might also help with the mind-body problem and the free will problem: How is the mind caused by the brain? How are free actions caused? Certainly this is an enormous gap in our understanding of nature, what with the ubiquity of causation, but a gap there seems little prospect of filling. The water may remain forever muddy. 
 I will be accepting the “skeptical realist” view of Hume in what follows, according to which causal powers are real existences that defy our limited understanding, not the positivist interpretation of Hume according to which the concept of causal power is incoherent and should be rejected.