Imagine, if you will, a civilization which has a lot to say about material objects, but much of what its major thinkers say about the subject is taken up with puzzling out and debating a major conundrum, which some of them consider one of the most important philosophical puzzles of all time: how could conservation of momentum, which is apparently not a material object, possibly interact with bodies in motion, which obviously are material objects? It is quite the problem. Some people, we will call them object dualists, hold that conservation of momentum is an object interacting with the material objects we call bodies; but it is an immaterial object rather than a material object. It was these people who started the whole discussion rolling. They had lots of people convinced for a while, but after a while people just couldn't figure out this whole interaction thing. How can conservation of momentum interact with material objects given that it can't touch them (we know it can't touch them because it is not a material object)? So, they reasoned, there's really only one alternative: conservation of momentum must actually be a material object, namely, a material object inside other material objects. This solves, or at least seems to solve, the interaction problem: at the very least, the interaction of bodies with conservation of momentum is no more mysterious than the interaction of bodies with bodies. Some people are not particularly impressed by this, thinking that the interaction of bodies with bodies is not so well understood, but the view does get a few followers.
The problems people have with this simple materialist solution, though, are fairly formidable, and others point out that (1) it really, really does seem like the object dualists are right that conservation of momentum isn't a material object; and (2) you can look and look at bodies all day and not find any material object in them that is clearly and obviously conservation of momentum. Admittedly, some people think that in bodies with motors the motor obviously must be the conservation of momentum, because it has something to do with momentum -- you can mess with the motor and change the momentum all over the place. The object dualists point out that this is just correlation, and is as true if conservation of momentum is an immaterial object interacting with material objects. But some of them concede that it may well be true that only bodies with motors, or at least motor-like things, really exhibit conservation of momentum; everything else that looks like conservation of momentum might not be.
Well, say some others, obviously the problem is that we haven't a clue what it is for anything but material objects to interact with material objects. But this doesn't matter. Obviously conservation of momentum can't itself interact with anything. But equally obviously, it's there, so what must be happening is that the material objects are causing conservation of momentum. Since conservation of momentum doesn't cause anything, not being a material object, but is an effect, it can't actually explain anything that happens in the real world; it's just a sort of by-product of things. And this solves the interaction problem; after all, we know that the interaction problem is the problem of how immaterial causes can have material effects, not the problem of how material causes can have immaterial effects. And it's supported by science, too; after all, material objects are constantly acting as if they are preparing to do things even though you never find the conservation of momentum already there causing them to do it, just as you would expect if conservation of momentum were really something that happened after bodies moved.
Hmmmph, others say, if you're going to go that far, you might as well just say there is no such thing as conservation of momentum. And others think that this is a good idea; obviously conservation of momentum is just a word put on our ignorance, and you can tell from all the philosophical trouble it's causing. Some day science will give us a theory which will explain everything without conservation of momentum; maybe we'll still use the phrase, maybe we won't, but if we do, it will just be a practical convenience: the true account of the universe will obviously not have any idea so spooky as conservation of momentum in it. But others respond, to all these epiphenomenalists and eliminativists, that the explanatory value of conservation of momentum seems well-established. Well, yes, of course, say some of the latter, if by that you mean that it enters into equations as a convenient calculating device sometimes. But, again, this is obviously just a practical convenience.
And so it goes. Always the discussion comes back to the paramount philosophical problem on this subject: either conservation of momentum must be an immaterial object, or it must be a material object; if it is a material object, it must either be a separate material object interacting with other material objects, or it just must somehow be each material object itself; and if the latter, we don't know how, but science obviously will someday tell us. Occasionally you have people who will suggest that conservation of momentum is not a material object, but is also obviously not an immaterial object, but, of course, if they were reasonable they would tell what kind of thing an object that is neither material nor immaterial could possibly be. And there are other people who say that interaction is just the wrong way to think of things: conservation of momentum is an immaterial principle, material objects act in such a way that their actions are explained by it, explanation does not require interaction, and, far from being two things in interaction, conservation of momentum and material objects aren't under any circumstances two things at all. Then others look at them strangely and say that, since the two can obviously be distinguished and things that are material and things that are not material are obviously not the same, if the latter explains anything about the former, we haven't avoided the interaction problem at all: if the immaterial principle explains anything about the material object, we are just back to the original question of how conservation of momentum, which is not material, interacts with material objects.
There are any number of science fiction and fantasy scenarios that could be run along similar lines, each one slightly different but all broadly analogous.
