Some of the country’s most famous investors, including Warren Buffett and John Bogle, have started to make the case that it’s time to dive back into the stock market.

They are usually careful to add that they don’t know what stocks will do in the short term. Yet their basic message is clear enough: stocks are now cheap, irrational fears have been driving the market down lately, and people who buy today will be glad that they did.

After a day like Tuesday, when the market rose 11 percent, it’s easy to see the merits of the argument.

But there is another argument that deserves more attention than it has gotten so far. It’s the bearish argument that is based neither on fears that the country may be sliding into another depression nor on gut-level worries about the unknown. It is based on numbers and history, and it has at least as much claim on reason as the bullish argument does.

It goes something like this: Stocks are truly cheap only relative to their values over the last 20 years, a period that will go down as one of the great bubbles in history. If you take a longer view, you see that the ratio of stock prices to corporate earnings is only slightly below its long-term average. And in past economic crises — during the 1930s and 1970s — stocks fell well below their long-run average before they turned around.

To make matters worse, corporate earnings have now started to plunge, too. Assuming that they keep dropping, stocks would also need to fall to keep the price-earnings ratio at its current level.

As stocks were soaring on Tuesday afternoon, I called James Melcher to hear a dose of fact-based bearishness. Mr. Melcher is president of Balestra Capital, a hedge fund in New York, who wrote an essay for his clients two years ago that predicted the broad outlines of the financial crisis (and then arranged Balestra’s portfolio accordingly). Like the bulls, he said that no one could know what the market would do in the short term. “But to think stocks are cheap now,” he added, “is not rational.”

He went on: “In the last 20 years — and particularly in the last six or seven — you had the most massive creation of liquidity the world has ever known.” Consumers went ever deeper into debt, thanks to loose lending standards, and a shadow banking system, made up of hedge funds and investment banks, allowed Wall Street to do the same. All that debt lifted economic growth and stock returns.

“It was a nice party,” Mr. Melcher said. “The problem is that all the bills are coming due at the same time.” He thinks stocks could easily fall an additional 20 percent and maybe 35 percent before hitting bottom.

So who’s right — the bears or the bulls? The smartest people in both camps, like Mr. Melcher, Mr. Buffett and Mr. Bogle, have a healthy dose of humility about their own conclusions. And when you dig into their arguments, you find that they’re not quite as different as they first sound. But they are different, and it’s worth taking a minute to consider the numbers.

There are any number of ways to measure the valuation of the stock market. Some examine prices relative to earnings, others are based on cash flow, a company’s underlying assets or the total value of the market. But they tell a pretty consistent story right now. Stocks, which were fabulously expensive for much of the 1990s and this decade, no longer are.

My favorite measure is the one recommended by Benjamin Graham and David L. Dodd, in their classic 1934 textbook, “Security Analysis.” They urged investors to use a price-to-earnings ratio — stock prices divided by average annual corporate earnings — based on at least five years of earnings and, ideally, closer to 10. Corporate profits may rise or fall in any given year, but a share of stock is a claim on a company’s long-term earnings and should be evaluated as such.

(Why not use a forecast of future earnings? Because they tend toward the fictional, as we’re now seeing once again.)

The 10-year price-to-earnings ratio tells an incredibly consistent story over the last century. It has averaged about 16 over that time. There have been long periods when it stayed above 16 and even shot above 20, like the 1920s, 1960s and recent years. As recently as last October, when other measures suggested the market was reasonably valued, the Graham-Dodd version of the ratio was a disturbing 27. But periods in which the ratio has jumped above 20 have always been followed by steep declines and at least a decade of poor returns.

By 1932, the ratio had fallen to 6. In 1982, it was only 7. Then, of course, the market began to self-correct in the other direction, and stocks took off.

After Tuesday’s big rally, the ratio was just a shade below 16, or almost equal to its long-run average. This is a little difficult to swallow, I realize. Stocks are down 40 percent since last October, and every experience from the last 25 years suggests they now have to bounce back.

But that’s precisely the problem. Since the 1980s, stocks have always bounced back from a loss, usually reaching a high in relatively short order. As a result, the market became enormously overvalued.

As Robert Shiller, the economist who specializes in bubbles, points out, human beings tend to put too much weight on recent experiences. We think the market snapbacks of 1987 and the current decade are more meaningful and more predictive than the long slumps of the 1930s, 1940s and 1970s. Of course, anyone who made the same assumption in 1930 or 1975 — this just has to turn around soon — would have had to wait years and years until the investment paid off.

Now, Mr. Buffett, Mr. Bogle and their fellow bulls know all this history, and they’re still bullish. (Though I’d be more bullish, too, if I could get the favorable terms that Mr. Buffett did. In exchange for his money and his good name, Goldman Sachs and General Electric each guaranteed him an annual return of at least 10 percent.)

So on Tuesday afternoon, I also called Mr. Bogle, the legendary founder of the Vanguard Group, the investment firm whose low-cost index funds have made a lot for a lot of people.

He, too, prefers the 10-year price-to-earnings ratio, he said, but he didn’t think that it necessarily had to fall to the same bargain-basement levels it reached in the 1930s and 1970s.

You can certainly see why that would be the case. Investors are well aware that the market fell to irrationally low levels during past crises, and they may not allow it to become so cheap this time around.

Mr. Bogle also thinks that corporate profits will rebound nicely within a couple of years and likes the fact that interest rates are low. Low rates have often — though not always — accompanied bull markets.

But it was his last argument that I think is the main one for most investors to focus on. “I’m not looking for a great bull market,” he said. There are some reasons to be optimistic about stocks, he said, “and I also look at the alternative.”

And, really, how attractive are the alternatives? Savings accounts and money market funds will struggle to keep pace with inflation. Bonds may, as well.

Stocks, on the other hand, are paying an average dividend of about 3 percent, which is better than the interest on many savings accounts, and stocks are also almost certain to rise over the next couple of decades.

If that is your time frame — decades, rather than months or years — this will probably turn out to be a perfectly good buying opportunity. In the shorter term, though, it’s a much tougher call, and it involves a lot more risk.

一些著名的投资家,包括沃伦.巴菲特,约翰.博格,公开表示,现在是返回股票市场买股票的时候了.虽然他们一如往常,小心翼翼的声明,他们并不能预知股票的短期走势.然而,他们透露的基本信息是明确的:股票价格现在很便宜,非理性的恐惧导致了近期市场走低,那些现在买入股票的人必将在今后得意于他们的选择.

本周二股市大涨11%,使上述观点轻易找到了论据.

但另外一种观点较之以前更值得注意.这就是依然看空市场.观点不是简单建立在国家经济将滑向另一场衰退的恐惧的情感上,也不是源自于对未知的极度担忧.看空的观点同样建立在数据与历史的分析基础上,至少,它的论据并不比看多观点的少.

看空者的部分看法如下:历史上看,一个巨大的经济泡沫的破裂后,需要经历20年的经济低迷期.以过去20年的数据看,股票价格确实比较便宜.但如果你在更长的历史周期上观察,你发现股价相对于公司赢利的比率只是略低于长期平均水平.回顾过去的经济危机----比如说发生于上世纪30年代和70年代的--- -那时的股价在反转前远远低于长期平均水平.

情况更糟的是,公司赢利水平开始恶化,如果假设赢利持续减少,股价也必须降低以维持当前的市盈率数值.

就在周二股市暴涨的时候,我致电James Melcher ,却听到了大堆有事实根据的看空理由.Melcher 先生是纽约的一家对冲基金,Balestra资本的主席,他曾经在2年前发文给他的客户,预测出金融危机可能情形(随后他又以此预测调整了Balestra的投资组合).如鼓励买股票的那些投资大家一样,他也认为没人能预测股市的短期走向.但他补充道:"现在认为股价很便宜是不明智的."

James Melcher 说道:"在过去20年中----尤其是过去6,7年----我们面临流动性泛滥,消费者大量举债,宽松的借贷标准,银行体系中不良机制,避险基金与投资银行的兴起,使华尔街大大提高了负债杠杆.负债刺激了经济,提高了股票回报."

"这是好的一面,"James Melcher 说,"问题在于所有的债务可能在同一时间到期."他认为股价很可能再跌20%,并且在见底前比现在下降35%.

看多与看空,谁是对的? Melcher ,Bogle,还有巴菲特,都是业界中的智者,他们的结论都经过认真的分析.当你深入他们的讨论,你会发现他们之间的分析并无太多的不同.然而却提出相反的观点.这就需要我们花点时间来研究一下历史数据了.

给股票估值有很多方法.有人用价格与盈利指标,有人用现金流折现,还有人考虑公司隐藏或低估的资产,或是公司市值.但现在使用那些方法只能如以前一样描述动人的故事.股价再不可能象上世纪九十年代与最近十年所经历的一样,不切实际的高高在上.

我个人喜好的一个衡量指标是,本杰明.格雷翰姆和戴维.多德在他们经典的1934年出版的教材<<证券分析>>里介绍的市盈率- ---股票价格除以过去5年的公司盈利平均值,计算的市盈率如果接近10,则是比较合理的.公司的利润水平在某些年份可能升也可能降,但股权是对公司长期盈利的分配要求,因此应该以平均的盈利水平来评估股价高低与否.(为什么不用预测的盈利数据计算市盈率?因为我们发现,那将可能导致数字的编造游戏)

以10年为周期的市盈率难以置信的揭示了一个世纪来的股市长期走势.在上世纪,市盈率的平均值是16,但有很长一段时间数值超过了16,甚至短时间内还大于20,如在上世纪的20年代,60年代和最近几年.就在去年10月,其他指标还显示股市还处于合理估值状态,格雷翰姆--多德指标却达到了令人不安的 27.如果一段时间内,市盈率超过20,伴随而来的情况是股市急剧回调,同时,至少在以后的10年内,投资回报都很差.

长期市盈率在1932年回落到6,1982年达到7,这之后,股市开始自我修正,股价反转拉升.

就在本周二的暴涨后,市盈率率低于长期均值16.这有点难以相信.股市从去年10月算起,已经下跌了40%,25年以来的经验也提示应该发生反弹了.但问题在于.自1980年以来,股价经常在略微下跌后就开始反弹,短期下跌后又创下新高,于是,整个市场已经严重高估了.

就如专门研究泡沫经济的经济学家罗伯特.席勒所言,人类更倾向于依据近期的经验来做决策.于是我们很容易预测股市就如1987年或最近的10年中发生的一样,很快就发生反弹,而不是类似二十世纪30,40,70年代经历漫长的低迷.当然,那些在1930年或1975年里认为股市即将转好的人,不得不等待很长的时间才让投资回本.

巴菲特与博格先生与其他唱多者都了解证券历史,但是他们还是看好后市.(如果我有巴菲特那样的优惠的交易条件,我也许比他们更愿意唱多市场.在高盛与通用电器的交易中,为了获得资金与巴菲特个人良好的声誉,两家公司都承诺每年给巴菲特至少10%的回报.)于是我在周二下午电话询问富有传奇色彩的先锋集团的创始人博格先生.先锋集团是一家投资公司,它掌管的低成本指数基金曾为很多人赚取大量财富.

博格先生说,他同样偏好于10年期的市盈率估值,但他不认为,指标要回到1930年或1970年的低值水平才开始购买股票.

你肯定能理解为何有这样的结论.投资者都充分意识到在以前的金融危机中市场曾跌到非理性的低价水平,于是在这次危机中他们可能不会让类似的低价再次出现.

博格先生同时认为,企业盈利水平在今后几年中很快得到恢复,他还注意到当前的利率水平很低.低利率经常伴随着牛市的出现----尽管不一定每次都是如此.

另外,要考虑其他的投资选择是否真正具有吸引力?储蓄,货币基金,以及债券,都必须要考虑抵消通涨的影响.而投资股票,平均能获得大约3%的股息率,超过了储蓄存款利率.而且就以后的几十年来说,股价的增长几乎是确定的.

如果你的投资期限放长到十年几十年,而不是几个月或几年,那么,现在可能是极好的买入机会.然而就短期而言,现在选择买入,结果可能很糟糕,同时还面临更大的投资风险.