In case you’re thinking that it’s safe to dive back into the stock market, think again. Jack Bogle, the father of the index fund, is advocating caution.

Over the past several years, index fund investors have reliably bought the dips in the stock market, expanding the valuations on growth stocks and helping push the S&P 500 higher. But that probably no longer is a savvy approach, says the founder of the Vanguard Group.