In a sense, the frequent refrain that China's debt is on an unsustainable path is true. After all, high levels of debt increase vulnerability to negative shocks. But, in another sense, this misses the point.

Many governments nowadays are accumulating debt in order to buttress public or private consumption. This approach, if overused, can amount to borrowing future demand; in that case, it is clearly unsustainable. But, if used as a transitional measure to help jump-start an economy or to provide a buffer from negative demand shocks, such efforts can be highly beneficial.

Moreover, in a relatively high-growth economy, ostensibly high debt levels are not necessarily a problem, as long as that debt is being used to fund investments that either yield high returns or create assets worth more than the debt. In the case of sovereign debt, the return on investment can be viewed as the increment to future growth.

The good news is that, in China, much of the accumulated leverage has indeed been used to fund investment, which in principle creates assets that will augment future growth. (Whether the results of the government's recent decision to increase the fiscal deficit to stimulate the economy follow this long-term growth-enhancing pattern remains to be seen.)

The bad news is that directed lending and the relaxation of credit standards in China, particularly after the crisis, have led to investment in assets in real estate and heavy industry with a value well below the cost of creating them. The return on them is negative.

China's so-called debt problem is thus not really a debt problem, but an investment problem. To address it, China must reform its investment and financial systems, so that low or negative return investments are screened out more reliably. That means tackling them is pricing of risk that results from the government's backing of the country's State-owned banks (which surely could not be allowed to fail).

Many developed countries are also failing to invest in high-return assets, but for a different reason: Their tight budgets and rising debts are preventing them from investing much at all. As this weakens growth and reduces inflation, the speed at which their sovereign-debt ratios can be reduced declines considerably.

In order to spur growth and employment, these economies must start paying closer attention to the kind of debt they accumulate. If the debt is financing growth-promoting investment, it may be a very good idea. If, however, it is financing "current operations" and raising short-term aggregate demand, it is highly risky.

Of course, the situation is not cut and dried. The return to public investment is affected by the presence or absence of complementary reforms, which vary from country to country. And there is some potential for abuse, with expenditures being misclassified as investments.

Yet, in an environment of low long-term interest rates and deficient short-term aggregate demand (which means there is little risk of crowding out the private sector), it is a mistake not to relax fiscal constraints for investment. In fact, the right kind of public investment would probably spur more private-sector investment. Identifying such investment is where today's debt debate should be.

The author, a Nobel laureate in economics, is professor of economics at New York University's Stern School of Business and senior fellow at the Hoover Institution. Project Syndicate