I'm reluctant to contribute to the Piketty backlash, as it seems to me to be mostly motivated by the unrealistic expectation that his book should have provided a comprehensive theory of everything. However this blog post from Alexander Douglas provides such a pithy account of the workings of public fiscal balances that it's worth recirculating. In response to the claim that "there are two main ways for a government to finance its expenses: taxes and debt," he writes:

Government spending isn’t financed by anything. The government pays for everything by crediting the non-government sector (employees, companies, foreign governments, etc.) with financial claims. Some of these claims are returned to the government in order to settle liabilities to the government (for instance in tax payments); others remain as financial holdings for the non-government sector. At any given moment the claims remaining as financial holdings constitute the whole of the ‘public debt’.

Tax revenue largely depends on the volume of spending. Decisions to spend rather than save are largely at the discretion of non-government agents. It is therefore very misleading to speak, as Piketty does, as if the government chooses to ‘finance its spending’ through taxation or debt. The amount of government spending that remains as ‘debt’ is largely up to the discretion of non-government agents choosing whether to hold onto financial claims or pass them on so that they can eventually find their way back to the government.

It therefore makes no sense to panic about government "budget" deficits, if you're not also going to bemoan private savings. Ironically, as it happens, private savings currently is a big problem, as corporations hand out mattress stuffing — in the form of dividends and share buybacks — rather than investing. Yet more ironically, the appropriate response is for the government to make up the investment shortfall through large fiscal deficits. Otherwise the economic stagnation rolls on until (sorry, I can't resist it) r>g.