It is actually a trifecta of problems. First, all of the city’s contracts with its employees expired years ago, the unions having calculated that they might fare better with a new, eager-to-please mayor than the lame-duck Bloomberg. You might think pay and benefits would be frozen in place when contracts lapse. You would be wrong. At this point in the conversation, Ravitch puts on his so-sorry-to-bore-you face and explains the Triborough Amendment. Under this 1982 legislative concession to public unions, the terms of the lapsed contract stay in place indefinitely — including the annual pay increases workers get as they accumulate seniority and move up the ladder. So public workers have little incentive to negotiate in tight times with parsimonious mayors. Most New York unions are holding out for 2 percent pay raises, retroactive, from the next mayor — a couple of billion dollars not provided for in the budget.

A bigger issue is health benefits. New York workers’ health insurance is paid by the city, which, according to the actuaries, has not salted away nearly enough money to cover the likely claims. The gap, known as an unfunded liability, is approaching an astounding $90 billion — $20 billion more than next year’s entire city budget.

And then there is the matter of pensions. While the private sector is rapidly converting to 401(k) plans, where the amount you get depends on how much the employer and employee have squirreled away over the years, New York City workers enjoy old-fashioned defined-benefit pensions, which guarantee a predetermined monthly sum for life. The government is supposed to set aside enough every year to cover its obligations. Calculating the amount the city needs to contribute toward future pensions is a matter of educated guesswork — and our city, like many others, has mostly, deliberately guessed low. After all, paying the prudent amount into the pension funds would mean either raising taxes (which alienates business and voters) or cutting spending (which costs union jobs). So everyone pretends that the pension funds’ brilliant investing strategy will pay bumper returns forever. (See this excellent account by Mary Williams Walsh and Danny Hakim.)

As a result, in the financial plan the next mayor inherits, the so-called uncontrollables (mostly pensions and health benefits governed by commitments made in the past) are, for the first time, greater than the controllables (the vast array of services and investments that we pay for in the annual budget).

Or, to pick another measure of our predicament, the total liabilities of the city now exceed its total assets by $125 billion, a condition referred to as balance-sheet insolvency.

“It’s something you mostly hear associated with struggling Rust Belt cities,” said E. J. McMahon, the veteran New York fiscal expert at the conservative Manhattan Institute. “It doesn’t mean you’re bankrupt, or that anything is happening soon. But it’s a warning sign.”

Ravitch, a Democrat who has worked on both sides of the business-labor divide, is not one of those scolds who demonize public employees. He believes that the moral obligation to pay workers their contractual due is as strong as the obligation to repay the bankers whose debt keeps the city afloat. He will tell you unions have behaved “very irresponsibly” at times, but he will also tell you that the idea of a taxed-to-death city that cannot afford to keep its promises is an exaggeration. “To sit in the Hamptons and listen to all the spoiled [bleeping] rich people complain about their taxes makes me want to throw up,” he told me. (This conversation took place over a table at a Hamptons restaurant thick with those spoiled rich people, lunching to the accompaniment of a strolling accordionist.)

These days most jurisdictions, given a choice between hitting up taxpayers and taking things back from workers, look for ways to throw workers under the bus. This can take the form of abruptly stopping payments of benefits to retirees (Stockton, Calif.), offloading retired workers onto the Obamacare insurance exchanges (Detroit and Chicago, among others, are contemplating that) or demanding concessions from current workers in the form of cost-sharing or reduced benefits (Wisconsin, after a long, rancorous battle).

In New York and many other places, the preferred option has been to kick the problem down the road. You do not have to be an actuary to marvel at the amount of flimflam politicians at all levels on all sides have employed to avoid facing the problem. My personal favorite: Are we having a hard time coming up with money for our annual payment into the New York State teachers’ pension fund? No problem. Thanks to a gimmick invented in Albany, we just borrow the money — from the pension fund!

Ravitch is aware of his reputation as a doomsayer but insists he is an optimist at heart. “I believe that our political system is going to fumble its way through,” he said. It has sometimes done so with his help, notably during the city’s near-bankruptcy of the 1970s. And, to be sure, there is an element of political ritual to the cycle of impending crisis and last-minute rescue. Governors and mayors, including Michael Bloomberg, like to go out with a balanced budget, so they raid every rainy-day fund and borrow to the hilt (and in Bloomberg’s case cut the police force by 5,000) to be models of fiscal diligence. Their successors come in, discover the cupboards are bare, and spend their electoral good will cobbling together a solution.

But while New York is in better shape than many other troubled states and localities, the situation is still insidious. Barring a sudden, unlikely return to economic boom times, the cost of retired workers will compete with the cost of everything else — including all the new promises made on the campaign trail.

And a number of factors compound the problem for the next mayor. The city cannot expect a lifeline, let alone adult guidance, from Albany, where Gov. Andrew Cuomo is already fending off appeals from other cash-strapped localities. Washington is not going to make things better and may very well make things worse. The Fed’s easy-money stimulus, a boon to New York, will not go on forever. And if Congress somehow overcomes its paralysis to do a serious budget deal, it is likely to shift new burdens to cities and states.

Ravitch told me he attended a dinner not long ago where the other guests included Paul Ryan, the author of the savagely austere Republican budget, and Erskine Bowles, the Democratic co-chairman of a bipartisan commission that produced a more centrist fiscal blueprint. Both plans, Ravitch noted, called for increasing the retirement age for Medicare from 65 to 67. So he asked the two budget authors, had they considered the fact that countless state and local government contracts ensure public employees health care until they are old enough for Medicare? “So while you are saving the federal government a lot of money, you are transferring the obligation to a much weaker set of tax bases,” Ravitch pointed out.

To which the response was, in effect: Not our problem.

You may believe that there is room for more taxing to close the gap. You may believe that it’s time for public employees to pay a larger share. You may believe, as Ravitch does, in some of each. The one thing you can’t say, if you’re the next mayor of New York, is: Not my problem.