Reasonable people may cordially disagree on the merits, as a measure, of the Dow Jones Industrial Average—which is in the news, this week, even more than usual after its return (however transient) to the zone above 10,000.

Yes, this threshold is arbitrary; worse, it's surprisingly slippery, thanks to the frequent changes in membership of the group of 30 stocks whose prices go into the calculation.

Regardless, let's take advantage of the huge mind-share of the DJIA, and use it as a lens through which to view the past and speculate on the future.

Calculating the DJIA is trivial on any given day, but it poses tricky questions if you want play "what if?" with alternate histories. I got into this game back in November 1999, when Microsoft and Intel were brought into the Group of 30 for the first time in place of Chevron and Union Carbide. In a PC Week column a few days later, I expressed displeasure with Dow Jones Corp. for chasing (as it seemed to me) the glitter and glitz of the tech-heavy NASDAQ Composite. (The latter index would peak, as we doubtless all remember, only four months later at a height that it has never since approached.)

I've since noted, for example in an eWEEK column in March 2005 (so far as I know, not available on line) that subsequent struggles of the DJIA would have been far less labored if Microsoft and Intel had not been holding it back. If we look back to December 2003, for example, when the DJIA crossed above 10,000 for the third time in its history, we can ask ourselves: what would have happened since then if Microsoft and Intel had been re-replaced by the stocks that were voted off the island four years earlier?

Others may approach this calculation differently, but as I compute this, today's DJIA would have topped 10,120 if MSFT and INTC had been sent back to the stock yard.

But wait, there's more. What if MSFT and INTC were replaced, not by two solid though unexciting petrochemical players, but instead by two exemplars of the cloud? Say, Amazon and salesforce.com? The latter did not go public until 2004, so this calculation has even more ways to do it than the one before—but as I can best run the numbers, a DJIA that had turned its eyes cloudward at its previous 10,000 breakthrough would today be nearing 10,760. We might not be dancing in the streets, but some of the parties that were held this week could have been more pleasant summer barbeques.

For investors, perhaps this mind game is a useful reminder that indicators aren't the same as measures. Sure, the DJIA is based on objective measures, but the continual re-composition of the DJIA massages those measures into well-informed opinions. There's nothing wrong with that, unless you misunderstand what the number means.

For everyone else, I offer this as an exercise in thinking about what will drive our economy forward during the decade to come. The "Wintel" model of thick-client computing had a good long run, but its day is nearly done. "We expect Windows and Office to be empty shells" a decade from now, says Toan Tran, an equity research analyst at Chicago's Morningstar Inc.

There's more to the market than simple numbers, but numbers are the basis of all real knowledge. Don't get caught in a shell game.