After the crash came the longest bull market in history. And now?

Shortly after the implosion of Lehman Brothers ten years ago, the actions began an epic race. The question now is whether this race is coming to an end.

Two of the world's leading economists and markets discussed this issue on Friday – and they were divided.

Robert Shiller, Yale economics professor, Nobel laureate, and expert in price evaluation of stocks and houses, took the bearish case.

He did not predict a crash, but he argued that equities seem expensive for many measures, including the CAPE ratio, a price-to-earnings comparison that takes into account multi-year profits, corrected for the past year. ;inflation.

This issue was popularized by Shiller himself and is often called Shiller PE. He said the ratio is high today and that this could be a sign that the market needs to calm down.

But Jeremy Siegel, a finance professor at the Wharton School of the University of Pennsylvania, said the shares did not seem so expensive, based on the last 12 months of profits.

He argued that shares are trading at a much lower valuation than they were at the height of the 2000 tech bubble. At that time, he and Shiller both feared the market to be sparkling.

Siegel pointed out that the S & P 500 is trading at about 18 times this year's operating profits, which can be rich but not too expensive.

He added that you also need to look at bond prices relative to equities – and hence, bonds are much more overvalued. Rates are close to historic lows.

The two men spoke Friday at a Wharton conference in New York on the impact of the 2008 financial crisis.

Shiller said he is concerned that the combination of rising stock prices and rising housing prices in many US cities could lead to another bubble.

But Siegel retorted that home prices around the world have risen faster than in many US markets. He does not worry about excessive foam in the real estate.

Shiller admitted that it's impossible to determine when the market will go down. This is the opposite of the weather forecast, he said. It is easier to predict the long-term performance of the market than what will happen in the next five days.

But the rally will end one day. Shiller said the risk of economic protectionism and trade tension around the world, not just in the US, could hurt the economy and markets.

And he said that it 's not clear how long – term stocks can benefit from Washington' s tax cuts, deregulation and assistance, from the US. as much as the political landscape always changes.

"Is Donald Trump permanent?" Shiller asked, causing a nervous laugh among conference participants.

CNNMoney (New York) First published on September 14, 2018: 1:10 pm ET