One of the things you hope students learn in a course like this is that money consists of three things: demand deposits (checking accounts and the like), currency and bank reserves. The first is a liability of private banks, the latter two are liabilities of the central bank. That money is always someone's liability -- a debt -- is often a hard thing for students to get their heads around, so one can end up teaching it a bit catechistically. Balance sheets, with their absolute (except for the exceptions) and seemingly arbitrary rules, can feel a bit like religious formula. On this test, the question about the definition of money was one of the few that didn't require students to think.
But when you do think about it, it's a very strange thing. What we teach as just a fact about the world, is really the product of -- or rather, a moment in -- a very specific historical evolution. We are lumping together two very different kinds of "money." Currency looks like classical money, like gold; but demand deposits do not. The most obvious difference, at least in the context of macroeconomics, is that one is exogenous (or set by policy) and the other endogenous. We paper this over by talking about reserve requirements, which allow the central bank to set "the" money supply to determine "the" interest rate. But everyone knows that reserve requirements are a dead letter and have been for decades, probably. While monetarists like Nick Rowe insist that there's something special about currency -- they have to, given the logic of their theories -- in the real world the link between the "money" issued by central banks and the "money" that matters for the economy has attenuated to imperceptible gossamer, if it hasn't been severed entirely. The best explanation for how conventional monetary policy works today is pure convention: With the supply of money entirely in the hands of private banks, policy is effective only because market participants expect it to be effective.
In other words, central banks today are like the Chinese emperor Wang Wei-Shao in the mid-1960s film Genghis Khan:
One of the film's early scenes shows the exquisitely attired emperor, calligraphy brush in hand, elegantly composing a poem. With an ethereal self-assurace born of unquestioning confidence in the divinely ordained course of worldly affairs, he explains that the poem's purpose is to express his displeasure at the Mongol barbarians who have lately been creating a disturbance on the empire's western frontier, and, by so doing, cause them to desist.
Today expressions of intentions by leaders of the world's major central banks typically have immediate repercussions in financial markets... Central bankers' public utterances ... regularly move prices and yields in the financial markets, and these financial variables in turn affect non-financial economic activity... Indeed, a widely shared opinion today is that central bank need not actually do anything. ...
In truth the ability of central banks to affect the evolution of prices and output ... [is] something of a mystery. ... Each [explanation of their influence] ... turns out to depend on one or another of a series of by now familiar fictions: households and firms need currency to purchase goods and services; banks can issue only reserve-bearing liabilities; no non-bank financial institutions create credit; and so on.
... at a practical level, there is today  little doubt that a country's monetary policy not only can but does largely determine the evolution of its price level..., and almost as little doubt that monetary policy exerts significant influence over ... employment and output... Circumstances change over time, however, and when they do the fictions that once described matters adequately may no longer do so. ... There may well have been a time when the might of the Chinese empire was such that the mere suggestion of willingness to use it was sufficient to make potential invaders withdraw.What looked potential a dozen years ago is now actual, if it wasn't already then. It's impossible to tell any sensible macroeconomic story that hinges on the quantity of outside money. The shift in our language from money, which can be measured -- that one could formulate a "quantity theory" of -- to discussions of liquidity, still a noun but now not a tangible thing but a property that adheres in different assets to different degrees, is a key diagnostic. And liquidity is a result of the operations of the financial system, not a feature of the natural world or a dial that can be set by the central bank. In 1820 or 1960 or arguably even in 1990 you could tell a kind of monetarist story that had some purchase on reality. Not today. But, and this is my point! it's not a simple before and after story. Because, not in 1890 either.
David Graeber, in his magisterial Debt: The First 5,000 Years , describes a very long alternation between world economies based on commodity money and world economies based on credit money. (Graeber's idiolect is money and debt; let's use here the standard terms.) The former is anonymous, universal and disembedded, corresponds to centralized states and extensive warfare, and develops alongside those other great institutions for separating people from their social contexts, slavery and bureaucracy.  Credit, by contrast, is personal, particular, and unavoidably connected with specific relationships and obligations; it corresponds to decentralized, heterogeneous forms of authority. The alternations between commodity-money systems,with their transcendental, monotheistic religious-philosophical superstructures; and credit systems, with their eclectic, immanent, pantheistic superstructures, is, in my opinion, the heart of Debt. (The contrast between medieval Christianity, with its endless mediations by saints and relics and the letters of Christ's name, and modern Christianity, with just you and the unknowable Divine, is paradigmatic.) Alternations not cycles, since there is no theory of the transition; probably just as well.
For Graeber, the whole half-millenium from the 16th through the 20th centuries is a period of the dominion of money, a dominion only now -- maybe -- coming to an end. But closer to ground level, there are shorter cycles. This comes through clearly in Axel Leijonhufvud's brilliant short essay on Wicksell's monetary theory, which is really the reason this post exists. (h/t
If I somehow found myself teaching this course in the 18th century, I'd explain that money means gold, or gold and silver. But by the mid 19th century, if you asked people about the money in their pocket, they would have pulled out paper bills, not so unlike bills of today -- except they very likely would have been bills issued by private banks.
The new world of bank-created money worried classical economists like Wicksell, who, like later monetarists, were strongly committed to the idea that the overall price level depends on the amount of money in circulation. The problem is that in a world of pure credit money, it's impossible to base a theory of the price level on the relationship between the quantity of money and the level of output, since the former is determined by the latter. Today we've resolved this problem by just giving up on a theory of the price level, and focusing on inflation instead. But this didn't look like an acceptable solution before World War II. For economists then -- for any reasonable person -- a trajectory of the price level toward infinity was an obvious absurdity that would inevitably come to a halt, disastrously if followed too far. Whereas today, that trajectory is the precise definition of price stability, that is, stable inflation.  Wicksell was part of an economics profession that saw explaining the price level as a, maybe the, key task; but he had no doubt that the trend was toward an ever-diminishing role for gold, at least domestically, leaving the money supply in the hands of the banks and the price level frighteningly unmoored.
Wicksell was right. Or at least, he was right when he wrote, a bit before 1900. But a funny thing happened on the way to the world of pure credit money. Thanks to new government controls on the banking system, the trend stopped and even reversed. Leijonhufvud:
Wicksell's "Day of Judgment" when the real demand for the reserve medium would shrink to epsilon was greatly postponed by regime changes already introduced before or shortly after his death [in 1926]. In particular, governments moved to monopolize the note issue and to impose reserve requirements on banks. The control over the banking system's total liabilities that the monetary authorities gained in this way greatly reduced the potential for the kind of instability that preoccupied Wicksell. It also gave the Quantity Theory a new lease of life, particularly in the United States.
But although Judgment Day was postponed it was not cancelled. ... The monetary anchors on which 20th century central bank operating doctrines have relied are giving way. Technical developments are driving the process on two fronts. First, "smart cards" are circumventing the governmental note monopoly; the private sector is reentering the business of supplying currency. Second, banks are under increasing competitive pressure from nonbank financial institutions providing innovative payment or liquidity services; reserve requirements have become a discriminatory tax on banks that handicap them in this competition. The pressure to eliminate reserve requirements is consequently mounting. "Reserve requirements already are becoming a dead issue."The second bolded sentence makes a nice point. Milton Friedman and his followers are regarded as opponents of regulation, supporters of laissez-faire, etc. But to the extent that the theory behind monetarism ever had any validity (or still has any validity in its present guises) it is precisely because of strict government control over credit creation. It's an irony that textbooks gloss over when they treat binding reserve requirements and the money multiplier as if they were facts of nature.
(That's more traditional textbooks. Newer textbooks replace the obsolete story that the central bank controls interest rates by setting the money supply with a new story that the central bank sets the interest rate by ... look, it just does, ok? Formally this is represented by replacing the old upward sloping LM curve with a horizontal MP (for monetary policy) line at the interest rate chosen by the central bank. The old story was artificial and, with respect to recent decades, basically wrong, but it did have the virtue of recognizing that the interest rate is determined in financial markets, and that monetary policy has to operate by changing the supply of liquidity. In the up-to-date modern version, policy might just as well operate by calligraphy.)
So, in the two centuries since Heinrich van Storch lectured the young Grand Dukes of Russia on the economic importance of "precious metals and fine jewels," capitalism has gone through two full Graeber cycles, from commodity money to credit money, back to (pseudo-)commodity money and now to credit money again. It's a process that proceeds unevenly; both the reality and the theory of money are uncomfortable hybrids of the two. But reality has advanced further toward the pure credit pole than theory has.
This time, will it make it all the way? Is Leijonhufvud right to suggest that Wicksell's Day of Judgment was deferred but not canceled, and now is at hand?
Certainly the impotence of conventional monetary policy even before the crisis is a serious omen. And it's hard to imagine a breakdown of the credit system that would force a return to commodity money, as in, say, medieval China. But on the other hand, it is not hard to imagine a reassertion of the public monopoly on means of payment. Indeed, when you think about it, it's hard to understand why this monopoly was ever abandoned. The practical advantages of smart cards over paper tokens are undeniable, but there's no reason that the cards shouldn't have been public goods just like the tokens were. (For Graeber's spiritual forefather Karl Polanyi, money, along with land and labor, was one of the core social institutions that could not be treated as commodities without destroying the social fabric.) The evolution of electronic money from credit cards looks contingent, not foreordained. Credit cards are only one of several widely-used electronic means of payment, and there's no obvious reason why they and not one of the ones issued by public entities should have been adopted universally. This is, after all, an area with extremely strong network externalities, where lock-in is likely. Indeed, in the Benjamin Friedman article quoted above, he explicitly suggests that subway cards issued by the MTA could just as easily have developed into the universal means of payment. After all, the "pay community" of subway riders in New York is even more extensive than the pay community of taxpayers, and there was probably a period in the 1990s when more people had subway cards in their wallets than had credit or debit cards. What's more, the MTA actually experimented with distributing subway card-reading machines to retailers to allow the cards to be used like, well, money. The experiment was eventually abandoned, but there doesn't seem to be any reason why it couldn't have succeeded; even today, with debit/credit cards much more widespread than two decades ago, many campuses find it advantageous to use college-issued smart cards as a kind of local currency.
These issues were touched on in the debate around interchange fees that rocked the econosphere a while back. (Why do checks settle at par -- what I pay is exactly what you get -- but debit and credit card transactions do not? Should we care?) But that discussion, while useful, could hardly resolve the deeper question: Why have we allowed means of payment to move from being a public good to a private oligopoly? In the not too distant past, if I wanted to give you some money and you wanted to give me a good or service, we didn't have to pay any third party for permission to make the trade. Now, most of the time, we do. And the payments are not small; monetarists used to (still do?) go on about the "shoe leather costs" of holding more cash as a serious reason to worry about inflation, but no sane person could imagine those costs could come close to five percent of retail spending. And that's not counting the inefficiencies. This is a private sales tax that we allow to be levied on almost every transaction, just as distortionary and just as regressive as other sales taxes but without the benefit of, you know, funding public services. The more one thinks about it, the stranger it seems. Why, of all the expansions of public goods and collective provision won over the past 100 or 200 years, is this the one big one that has been rolled back? Why has this act of enclosure apparently not even been noticed, let alone debated? Why has the modern equivalent of minting coinage -- the prerogative of sovereigns for as long as there've been any -- been allowed to pass into the hands of Visa and MasterCard, with neoliberal regimes not just allowing but actively encouraging it?
The view of the mainstream -- which in this case stretches well to the left of Krugman and DeLong, and on the right to everyone this side of Ron Paul -- is that, whatever the causes of the crisis and however the authorities should or do respond, eventually we will return to the status quo ante. Conventional monetary policy may not be effective now, but there's no reason to doubt that it will one day get back to so being. I'm not so sure. I think people underestimate the extent to which modern central banking depended on a public monopoly on means of payment, a monopoly that arose -- was established -- historically, and has now been allowed to lapse. Christina Romer's Berkeley speech on the glorious counterrevolution in macroeconomic policy may not have been anti-perfectly timed just because it was given months before the beginning of the worst recession in 70 years, but because it marked the end of the period in which the body of theory and policy that she was extolling applied.
 Information wants to be free. If there's a free downloadable version of a book out there, that's what I'm going to link to. But assuming some bank has demand deposits payable to you on the liability side of its balance sheet (i.e. you've got the money), this is a book you ought to buy.
 In pre-modern societies a slave is simply someone all of whose kinship ties have been extinguished, and is therefore attached only to the household of his/her master. They were not necessarily low in status or living standards, and they weren't distinguished by being personally subordinated to somebody, since everyone was. And slavery certainly cannot be defined as a person being property, since, as Graeber shows, private property as we know it is simply a generalization of the law of slavery.
 A point also emphasized by Robert Triffin in his essential paper Myths and Realities of the So-Called Gold Standard.
 Which is a cautionary tale for anyone who thinks the fact that an economic process that involves some ratio diverging to infinity is by defintion unsustainable. Physiocrats thought a trajectory of the farming share of the population toward zeo was an absolute absurdity and that in practice it could certaily not fall below half. They were wrong; and more generally, capitalism is not an equilibrium process. There may be seven unsustainable processes out there, or even more, but you cannot show it simply by noting that the trend of some ratio will take it outside its historic range.
UPDATE: Nick Rowe has a kind of response which, while I don't agree with it, lays out the case against regarding money as a liability very clearly. I have a long comment there, of which the tl;dr is that we should be thinking -- both logically and chronologically -- of central bank money evolving from private debt contracts, not from gold currency. I don't know if Nick read the Leijonhufvud piece I quote here, but the point that it makes is that writing 100-odd years ago, Wicksell started from exactly the position Nick takes now, and then observed how it breaks down with modern (even 1900-era modern) financial systems.
Also, the comments below are exceptionally good; anyone who read this post should definitely read the comments as well.