(Reuters/Brendan McDermid)

On Wednesday, Federal Reserve Chair Janet Yellen said that even though "risks to financial stability are moderated," stock prices are still "quite high" right now. Markets, though, shrugged off this concern. The Standard & Poor's 500 was only down 0.78 percent on the day.

If this is our "irrational exuberance" moment, it's not much of one — not that the original one was either. That came back on Dec. 5, 1996, when, in the middle of an otherwise unremarkable speech, then-Fed Chair Alan Greenspan asked "how do we know when irrational exuberance has unduly escalated asset values, which then become subject to unexpected and prolonged contractions as they have in Japan the past decade." Japan's stock market, as economist Robert Shiller points out, promptly fell 3 percent, Hong Kong's did too, and then London and Frankfurt's dropped 4 percent. The U.S. stock market opened the next day down 2 percent, but rallied thereafter. And then it kept rallying for the next three years, before the boom turned into a bubble.

Timing the market, in other words, isn't easy even if you're the Fed chair. Maybe that's why Greenspan didn't try again. But after the housing market's collapse nearly brought the entire global economy with it, the Fed realized that it couldn't blithely assume it could ignore what was going on and just clean up any financial mess. It'd have to do what it could—either talking down markets or regulating them or both—to keep the financial system safe without sacrificing the economy. That's why Yellen warned last summer that "valuation metrics" for "smaller firms in the social media and biotechnology industries"—aka startups—"appear substantially stretched." That only set off a short-lived sell off, though.

Now, stocks have cooled off since the start of the year, but not that much. So does that mean stock prices are "quite high"? Well, that depends on how you look at it. Take Robert Shiller's cyclically-adjusted price-earnings ratio, or CAPE, which looks at the past ten years of earnings to figure out how pricey stocks are today. The idea here is that it smooths out any big ups or downs, and shows us how fairly valued—or not—stocks are. And by this measure, as you can see below, stocks really are getting expensive.

Source: Robert Shiller

The only problem is they've been getting expensive for awhile now. "According to CAPE," Crossing Wall Street's Eddy Elfenbein told me, "stocks have been valued above average for most of the last 25 years." Part of that is accounting standards are different than they used to be, so valuations are too—they're higher. Another part is that interest rates having been falling the last three decades, and, all else equal, lower rates should mean higher stock valuations. And the last part is that CAPE overweights what happened before to what's happening now. Think about it like this. Today's 27.2 CAPE ratio is so high, in part, because earnings were so bad during the financial crisis. But it's a little funny to say that a historically crummy economy seven years ago means stocks are overvalued now.

Another way to look at this is to just consider last year's earnings. Now this has the opposite problem of only weighting what's happening now. So if earnings are negligible or negative, like they were in 2008, this will say that stocks are super expensive when they're actually super cheap. But as long as we keep that in mind, this still helps us look at stocks from a slightly different angle. And it tells us that, with a PE ratio of 19.7, the S&P 500 isn't a bargain, but it isn't exorbitant either. In other words, it's a little high, but compared to the last 25 years, not crazily so. (That'd be the tech bubble). Besides, it's a "misperception that the market falls due to valuation," Elfenbein says, when "more often stocks fall with lower fundamentals instead of prices soaring beyond fundamentals."

For now, at least, those fundamentals are, well, okay. Sure, stock prices are elevated, but so are earnings. And really, where else are you going to put your money? Bank accounts barely offer any interest, and bonds are about as pricey as they could possibly be. That said, it's true that stocks have outpaced bonds enough that you should be at least a little wary.

So stocks might not be quite high, but that depends on your definition of "quite." It's worth worrying about, and it's a good thing that the Fed is reminding us of that.