While billionaire Warren Buffett and best-selling author Michael Lewis approach the stock market from different perspectives, they're both brilliant and know a thing or two about Wall Street. It's for this reason that investors would be wise to listen when they offer identical advice.

To most people, Buffett, the chairman and CEO of Berkshire Hathaway (BRK.A -0.30%) (BRK.B -0.26%), represents proof that beating the market is possible given sufficient intelligence and the right temperament.

But while this may or may not be true, it doesn't mean you should try to beat it.

Unless you're willing to devote your life to investing and reading literally hundreds of pages of regulatory filings a day -- when asked how to get smarter, Buffett once held up stacks of paper and said he "read 500 pages like this every day" -- your best alternative is to do one thing: buy a low-cost index fund that tracks the broad market.

Don't take it from me; here are the instructions Buffett laid out to the trustee of his will:

Put 10% of the cash in short-term government bonds and 90% in a very low-cost S&P 500 (^GSPC -0.46%) index fund. (I suggest Vanguard's.) I believe the trust's long-term results from this policy will be superior to those attained by most investors -- whether pension funds, institutions, or individuals -- who employ high-fee managers.

For the record, the fund he's referring to is Vanguard's S&P 500 (VOO -0.41%) -- to read more about this simple and effective approach to investing, check out my colleague Jordan Wathen's take on it here. 

Coming at it from a different angle, Lewis offers similar advice. Lewis' perspective was formed through a short stint at Salomon Brothers in the 1980s and through decades of researching and writing about Wall Street.

His most recent book, Flash Boys, looks at the role high-frequency trading plays in the manipulation -- or, in Lewis' words, "rigging" -- of the public equity markets.

Asked in a recent interview how he manages his own considerable net worth, here's his response (emphasis added):

I don't have an advisor. I bought some municipal bonds a few years ago and I had someone who actually just specializes in California municipal bonds do that for me.

But other than that, my financial life is really dull. I either put it in index funds or I give it to Warren Buffett to invest. Let him worry about it.

The point I'm trying to make is that some of the world's brightest financial minds advise investors to take the simplest of approaches. It's a lesson I've discussed before, and it's one that can't be given enough press.