We are very near the end, but have not and will not lose our good cheer. -- Robert Falcon Scott

NEW YORK ( TheStreet) -- Having run out of credible reasons for why U.S. equities should continue to advance at their current pace with no intervening correction, the bulls are down to their final argument: This is not yet a 1999-type bubble.

You have likely heard some variation of this thesis argued over the past month. Essentially, the case being made is that because the stock market has yet to reach the insane levels of the historic bubble in 1999 to 2000, we have much more room to run.

In looking at a chart of long-term valuation metrics such as the Shiller P/E, it is not difficult to see why the bullish thesis has shifted. The current Shiller P/E of 24.4 is higher than 91% of historical time periods.

With the exception of brief periods in 1901 and 1929, the only time the Shiller P/E has been higher than today was during the 1996 to 2000 bubble and the 2003 to 2007 bull market. As only a 13% advance from current S&P levels would bring the Shiller P/E up to the 2007 peak level, to be extremely bullish here you really need to be betting on another 1996 to 2000 bubble.

Many bulls will argue that there is a good chance of this occurring given the loosest monetary policy in history and the Federal Reserve's stated policy of inflating stock prices in order to create a "wealth effect." While I cannot refute that there is indeed a chance of this occurring, I would certainly not be saying the chance is high and advising investors to bet on this outcome. There have been many bull markets but there was only one that reached the extreme levels of 2000. A repeat performance just 13 years later is highly unlikely.

The reason for this is simple. Investors are emotional beings and the 2000 bubble and subsequent crash (50% decline in the S&P 500, 78% decline in the Nasdaq) is still fresh in the minds of many. For another bubble to develop, you generally need enough time to have passed for a new generation of investors to take over who have no painful memories over the prior crash. That is why it is rare to have a bubble in the same asset class just 13 years apart.

Still, some bulls might argue that even if another bubble is not likely they would happily stay invested for another 13% upside that would bring the S&P back to 2007 peak valuation levels. This is fine assuming these bulls have the uncanny ability to get out at the top. Remember, from October of 2007 to March of 2009, the S&P 500 declined 57%, giving back all of the gains from 2003 to 2007 advance and trading back to 1996 levels.

Overall, bulls are reaching here in calling for another bubble as a justification for gains to continue at a similar pace to this year. This not atypical for late-stage, high valuation bull markets as thesis creep inevitably takes hold when data-driven rationales are no longer valid. U.S. equities may very well continue higher here as momentum remains strong and valuation is not a great predictor of short-term returns. But looking out further, unless this is indeed a repeat of 1996 to 2000, investors are likely to be highly disappointed.

At the time of publication, the author held no positions in any of the stocks mentioned.