Warren Buffet is such a firm believer in this principle that he bet a hedge fund manager $1 million that he couldn't beat the S&P over a decade[0]. Buffet won. The S&P beat the manager by 5% ANNUAL return.
Hedge fund managers pick stocks all day, every day. If these guys with Ivy League finance bona-fides aren't able to consistently beat the S&P, you should really think about whether you'll be able to beat it in your spare time.