It's a general rule that the internal logic of a system only becomes visible when it breaks down. A system that is smoothly reproducing itself provides no variation to show what forces it responds to. Constraints are invisible if they don't bind. You don't know where power lies until a decision is actively contested.
In that sense, the crises of the past seven years — and the responses to them — should have been very illuminating, at least if we can figure out what to learn from them. The current crisis in Greece is an ideal opportunity to learn where power is exercised in the union, and how tightly the single currency really binds national governments. Of course, we will learn more about the contours of the constraints if the Syriza government is more willing to push against them.
The particular case I'm thinking of right now is our conventional language about central banks "printing money," and the related concept of monetary sovereignty. In periods of smooth reproduction we can think of this as a convenient metaphor without worrying too much about what exactly it is a metaphor for. But if Greece refuses to accept the ECB's conditions for continued support for its banks, the question will become unavoidable.
We talk about governments "printing money" as if “money” always meant physical currency and banks were just safe-deposit boxes. Even Post Keynesian and MMT people use this language, even as they insist in the next breath that money is endogenously created by the banking system. But to understand concretely what power the ECB does or does not have over Greece, we need to take the idea of credit money seriously.
Money in modern economies means bank liabilities.  Bank liabilities constitute money insofar as a claim against one bank can be freely transferred to other units, and freely converted to a claim against another bank; and insofar as final settlement of claims between nonfinancial units normally takes the form of a transfer of bank liabilities.
Money is created by loan transactions, which create two pairs of balance-sheet entries — an asset for the borrowing unit and a liability for the bank (the deposit) and a liability for the borrowing unit and an asset for the bank (the loan). Money is destroyed by loan repayment, and also when the liabilities of a bank cease to be usable to settle claims between third parties. In familiar modern settings this lack of acceptability will be simultaneous with the bank being closed down by a regulatory authority, but historically things are not always so black and white. In the 19th century, it was common for a bank that ran out of reserves to suspend convertibility but continue operating. Deposits in such banks could not be withdrawn in the form of gold or equivalent, but could still be used to make payments, albeit not to all counterparties, and usually at a discount to other means of payment. 
To say, therefore, that a government controls the money supply or "prints money" is simply to say that it can control the pace of credit creation by banks, and that it can can maintain the acceptability of bank liabilities by third parties — which in practice means, by other banks. It follows that our conventional division of central bank functions between monetary policy proper (or setting the money supply), on the one hand, and bank regulation, operation of the interbank payments system, and lender of last resort operations, on the other, is meaningless. There is no distinct function of monetary policy, of setting the interest rate, or the money supply. "Monetary policy" simply describes one of the objectives toward which the central bank's supervisory and lender-of-last-resort functions can be exercised. It appears as a distinct function only when, over an extended period, the central bank is able to achieve its goals for macroeconomic aggregates using only a narrow subset of the regulatory tools available to it.
In short: The ability to conduct monetary policy means the ability to set the pace of new bank lending, ex ante, and to guarantee the transferability of the balances thus created, ex post.
It follows that no country with a private banking system has full monetary sovereignty. The central bank will never be able to exactly control the pace of private credit creation, and to do so even approximately except by committing regulatory tools which then are unavailable to meet other objectives. In particular, it is impossible to shift the overall yield structure without affecting yield spreads between different assets, and it is impossible to change the overall pace of credit creation without also influencing the disposition of credit between different borrowers. In a system of credit money, full monetary sovereignty requires the monetary authority to act as the monopoly lender, with banks in effect serving as just its retail outlets. 
Now, some capitalist economies actually approximate to this pretty closely. For example the postwar Japanese system of “window guidance” or similar systems in other Asian developmental states.  Something along the same lines is possible with binding reserve requirements, where the central bank has tight operational control over lending volumes. (But this requires strict limits on all kinds of credit transactions, or else financial innovation will soon bypass the requirements.) Short of this, central banks have only indirect, limited influence over the pace of money and credit creation. Such control as they do have is necessarily exercised through specific regulatory authority, and involves choices about the direction as well as the volume of lending. And it is further limited by the existence of quasi-bank substitutes that allow payments to be made outside of the formal banking system, and by capital mobility, which allows loans to be incurred, and payments made, from foreign banks.
On the other hand, a country that does not have its “own” currency still will have some tools to influence the pace of credit creation and to guarantee interbank payments, as long as there is some set of banks over which it has regulatory authority.
My conclusion is that the question of whether a country does or does not have its own currency is not a binary one, as it's almost always imagined to be. Wealth takes to form of a variety of assets, whose prospective exchange value can be more or less reliably stated in terms of some standard unit; transactions can be settled with a variety of balance-sheet changes, which interchange more or closely to par, and which are more or less responsive to the decisions of various authorities. We all know that there are some payments you can make using physical currency but not a credit or debit card, and other payments you can make with the card but not with currency. And we all know that you cannot always convert $1,000 in a bank account to exactly $1,000 in cash, or to a payment of exactly $1,000 – the various fees within the payment system means that one unit of “money” is not actually always worth one unit. 
In normal times, the various forms of payment used within one country are sufficiently close substitutes with each other, exchange sufficiently close to par, and are sufficiently responsive to the national monetary authority, relative to forms of payment used elsewhere, that, for most purposes, we can safely speak of a single imaginary asset “money.” But in the Greek case, it seems to me, this fiction obscures essential features of the situation. In particular, it makes the question of being “in” or “out of” the euro look like a hard binary, when, in my opinion, there are many intermediate cases and no need for a sharp transiton between them.
 Lance Taylor, for instance, flatly defines money as bank liabilities in his superb discussion of the history of monetary thought in Reconstructing Macroeconomics.
 Friedman and Schwartz discuss this in their Monetary History of the United States, and suggest that if banks had been able to suspend withdrawals when their reserves ran out, rather than closed down by the authorities, that would have been an effective buffer against against the deflationary forces of the Depression.
 Woodford's Interest and Prices explicitly assumes this.
 Window guidance is described by Richard Werner in Masters of the Yen. The importance of centralized credit allocation in Korea is discussed by the late Alice Amsden in Asia's Next Giant.
 Goodhart's fascinating but idiosyncratic History of Central Banking ends with a proposal for money that does not seek to maintain a constant unit value – in effect, using something like mutual fund shares for payment.