Keynes identified what he called the "paradox of thrift."  He was studying demand-side economic shocks essentially.  If you consider what Dan says above about supply and demand intersecting and creating a market clearing price, Keynes was trying to understand how that system could break down on a massive scale.

So, think about how when we entered this recession (depression?), we heard a lot about how we have to "tighten our belts" and "take our medicine" in order for markets to clear (heard more than a bit of that here at Dag even).  Keynes realized that this was a fallacy of composition.  Even though buckling down and being frugal when times are tough makes all the sense in the world to an individual, when everyone does it at once it makes things worse. If your only solution is belt-tightening, you're going to end up feeling like you're getting the business from a boa constrictor.

So, Keynes said that the thing to do was to stop this cycle by essentially just giving money to people who would spend it.  People have immediate needs - food, clothing, shelter - that they will spend money on if they have it.  This puts other people back to work and hopefully leaves a vicious cycle behind for a virtuous one.

Keynes argued for direct fiscal stimulus as a result.  You may have heard his famous (infamous?) plan to dig a hole, bury money in it, and then pay people to dig it up.  Contemporary Neo-Keynesians have argued that this money would be best spent on things that we actually need, like our crumbling infrastructure.  Another policy implication from Keynes is unemployment insurance.  If a robust social safety net is place, people who lose their jobs won't stop spending money, which results in other people losing jobs, and the cycle wouldn't have a chance to get so vicious.

The other major school of thought, at least in contemporary American economics, is monetarism, which is more or less from Milton Friedman.  Keynes lost his luster in the 1970s when stagflation reared it's ugly head.  According to old school Keynesian theory, you just couldn't have rising inflation at the same time you had rising unemployment.  For instance, right now you have basically no inflation, possibly even deflation, and high unemployment.  That's classic Keynes.  But there were two big differences in the 1970s.  One was that you had some considerable supply shocks - oil in particular - and the other was that you had a new actor on the scene: the central bank.

The monetary view is not actually all that different than the Keynesian view.  It more or less supports what the Fed has been doing by trying to expand the money supply.  Friedman argued that the central bank could essentially smooth out recessions by either expanding or contracting the money supply.  However, "trying" is the key word presently. As you might have read, much of the money the Fed is releasing is simply piling up in banks.  If you want to know why, MMT is a great way to understand.

There are other views, sometimes referred to as heterodox since the freshwater (Friedman) and saltwater (Keynes) views tend to dominate contemporary American economics in academia.  One is the Austrian view.  That's where Ron Paul is at.  The problem for the Austrians is that they tend to eschew mathematics as a tool.  Old school Austrians preached what they called "methodological individualism" and argued that economics should more or less be practiced like classical philosophy: a priori reasoning leading to axiomatic systems of thought.  The problem with this is that their axiomatic system ends up predicting that employment should increase during the "bust" phase of their model.  Empirically, this does not seem to occur.  In any case, the Austrian prescription for recession is to go ahead and let deflation do its thing.  Seriously.

Thinking these three views over, it occurs to me that the powers that be in US and EU are actually closest to the Austrians in their response to the financial collapse of 2008.  At this point, I would even go for some sincere Friedman, but that guy is probably a raving socialist by contemporary standards.

Anyhow, hope some that made sense.

Your first graf describes observations, not causal mechanisms.  Newton observed the interaction of forces, but he didn't actually explain the causal interaction. If I push on a wall, why doesn't my hand go through it?  He wouldn't have really known how to answer that question, except maybe perhaps to just deflect it by talking about "solids."  Newton didn't have an understanding of what we now call the normal force.  And he was very dissatisfied with the role gravitation played in his framework, but couldn't seem to do better than "action at a distance."  I guess you can't fault him for that, because no one since has cracked that walnut. EDIT: This reads a bit sloppy to me, but I'm going to leave it as is.  The point simply this: Newton did not have causal explanations for things like gravity or the normal force.  He had an observational account that he turned into a theoretical model - a theoretical model that has been proven not to be correct except for within the limited scope of "human scale" physical interactions.  And the whole reason I'm raising that is because you say you have a problem with economics because it doesn't offer mechanistic causal explanations.  Well, neither did Newton.  For that matter, how about Kepler?  Kepler's law is true, but he didn't even have a theory for it, much less a mechanistic explanation - except the data supported it.  This is not at all unlike many of the findings of modern economics - Okun's law for example.

But the point is, you were criticizing modern economics for failing to be about "causal narratives."  You said it should explain mechanisms of causal action.  I agree, but I think that's exactly what modern economics seeks to do.

I don't really care much for your usage of "practical art."  Art is human expression.  Science is human understanding.  That doesn't mean those things don't interact, but they aren't interchangeable.  Understanding ​how​ an engine works, in terms of being able to describe its operation, is not at all the same thing as understanding why​ it works - in other words, understanding things like thermodynamics, thermal expansion, compression of gasses, electricity, etc.  You don't actually need to know the theoretical underpinnings any of that stuff to describe the process of an engine working.  But fixing the damned thing is no art.  I fix and ride old motorcycles.  There might be art in the ride, but making sure the thing runs right could be the difference between a good ride and a seriously bad day.  You can bet I approach that task as a scientific endeavor, demanding precise measurement and thorough testing, not as an exercise in expression or imagination.

Unfortunately, your description of the task just being too complex and "physics envy" sounds a bit too Austrian to me.  It's basically the same way Mises and Hayek used to talk about their counterparts.  It's no more true now then it was then.  In fact, much less so.

RE: who speaks for MMT, I have no idea.  I do know that Galbraith, who seems to be regarded as one of the leading proponents, has gone back and forth on this with Krugman over the past couple of years and the end result was that they agreed on what should be done.  If you think Krugman, DeLong, et al. wouldn't want what you're asking for, I'm forced to conclude you just don't follow them very closely.  Pretty much the entire saltwater school was howling for more during the first two years of Obama's term.  You may have noticed that since then we've composed an even less cooperative Congress.  To the extent that those guys have demurred in terms of what they're calling for, it's because they did it for two years to no avail.  So now they're focusing on advocating initiatives that a) have some chance of helping and b) have some chance of happening.  That doesn't mean they wouldn't welcome circumstances that were conducive to much bolder action.

FWIW, the differences that you mention don't seem very important to me.  Again, they strike me as more or less semantic and primarily descriptive.  No, the fractional reserve banking story that is taught to undergrads isn't accurate, but it gets them beyond thinking of money as green pieces of paper.  They don't teach the double-book story in grad school.  And that's why New-Keynesian PhDs don't seem to find any of what the MMTers are asking for the be controversial.  They agree that we can afford it.  They agree that QE funds are piling up in the banking system.  Again, I wonder how diligently you actually read Krugman et al., because he certainly doesn't think QE might work because of hitting some magic money supply number.  He thinks it will work if and only if the Fed can use it to credibly create credible inflation.  Go back and read the exchange between Krugman and Galbraith on this.  There is no daylight between them on this point.  For their small disagreements on how inflation might eventually show up, they both agree that we are nowhere near enough to that point to be concerned and should actually welcome 4 or 5 percent.

Finally, all of this has caused me to rethink what you said about comparative statics in the first place.  The thing is, the simple supply and demand graphs that are used to teach freshman are just the beginning.  It's the first baby step in trying to understand a system that is complex and dynamic.  But here's the thing: they're derived from analytical relationships that in most cases have some pretty strong empirical evidence.  Not only that, but they are used ​precisely to build causal narratives​.  They help you understand how things move if supply changes or demand changes or a tax is introduced or whatever you might want to entertain.  They're not temporal because they're not supposed to be.  It's a starting point, not the totality or even the crowning achievement of economics.  And it's only one tool of many.

All in all, it almost seems like you believe in some kind of mainstream economic dogma that I'm not sure exists.  This stuff isn't lost on practicing economists (except when it is).  Basically nothing that you're complaining about is found beyond freshman courses and even then it is not taught or regarded, at least in my experience, as you seem to think.  IMHO, much of what you've written here seems to misrepresent what is in economics textbooks, what's being taught in courses and what practicing economists are actually out there saying.

I'm sorry if that comes off too prickish.  I'm really not trying to be rude or offensive.  I've just spent too much time studying this stuff not to defend the parts of it that I think are worthwhile.  I just don't think your description of modern econ, as an academic discipline, is all that accurate.  It may be true of some people (Fama, Cochrane and Taylor come immediately to mind, though I think they say what they because of the political implications more than because they don't understand what's going on), but I don't think it's a very fair characterization in general.

Economics is not perfect.  And it shouldn't simply ape physics or any other discipline.  What it should do is follow the scientific method: observe, theorize, test.  That has always helped to increase understanding.  The answer is not to somehow sever it or compartmentalize it from science as some "practical art."  The answer is more science.  That doesn't necessarily mean more mathematical complexity.  It means an honest and disciplined cleavage with what is, with empirical reality.  Like all other scientific disciplines, genuine progress depends on Feynman-esque "bending over backwards" to be honest about methodology, findings, claims to knowledge.. everything.