I'd like to start talking about scientific explanation here. This is the particular problem I have been working on recently:

A plausible hypothesis is that scientific explanations are answers to "why" questions about phenomena. If I hear a "cawing" noise and I ask my friend why I hear this cawing. This is a familiar enough situation that most of us would have our curiosity satisfied by an answer as simple as "there is a crow". But say the situation was unfamiliar (perhaps the question is asked by a child). In that case "there is a crow" is unsatisfactory. It is unsatisfactory even if "Sometimes, crows caw" is a universal regularity of nature. All we've done is conjoined a noise (cawing) to an object (the crow). One reason we might not find this to be a good explanation is that it is a "curiosity stopper", like answering "electricity!" to the question of "Why does flipping a switch turn a light bulb on?". But the problem is worse than that because "Sometimes, crows caw" actually does allow you to make predictions in the way "electricity!" does not. We could even posit as true the law that "Crows always caw and only crows caw" and get extremely firm predictions-- but because we are still just conjoining objects and events we aren't really understanding anything.

Of course we can say more about crows and cawing. We can talk about the crow's voice box and vibrations in the air which vibrate hair fibers which we process as sound. But of course this explanation is just like the first one. We are conjoining objects and events (lungs, blowing air, voice box shape structuring vibrations, vibrations moving throw the air, air vibrating in the cochleae). For almost everyone this explanation (written out less haphazardly than I have) would appear to be a fairly complete explanation. But it has exactly the same problems as the first explanation (though it is longer and perhaps includes more generally applicable laws).

Now obviously this explanation can be extended further, right down to quantum theory. But even this explanation (if it could ever be written out) would include unreduced terms that are just conjoined to each other through natural laws. And we can still ask why questions about fundamental particles and their behavior. Yet we want to say that a quantum based explanation of crow cawing would be complete (or at least that there is some theory sufficiently fundamental that it could be used to give a complete explanation of the cawing noise).

Yet it looks to me like even the most fundamental explanation will still be just a list of conjoined events and that we will still be able to ask why questions about these events and their relations. We either need to be able to point to a special class of "complete" explanations and say why they qualify for this class OR we need to give an account of non-complete explanations that tells us why we really are understanding events better when we get them.