Successful investing is all about common sense

Successful investing is all about common sense. As the Oracle has said, it is simple, but it is not easy. Simple arithmetic suggests, and history confirms, that the winning strategy is to own all of the nation’s publicly held businesses at very low cost. By doing so you are guaranteed to capture almost the entire return that they generate in the form of dividends and earnings growth.

The best way to implement this strategy is indeed simple: Buying a fund that holds this market portfolio, and holding it forever. Such a fund is called an index fund. The index fund is simply a basket (portfolio) that holds many, many eggs (stocks) designed to mimic the overall performance of any financial market or market sector. 

Classic index funds, by definition, basically represent the entire stock market basket, not just a few scattered eggs. Such funds eliminate the risk of individual stocks, the risk of market sectors, and the risk of manager selection, with only stock market risk remaining (which is quite large enough, thank you). Index funds make up for their short-term lack of excitement by their truly exciting long-term productivity.

John Bogle - The Little Book of Common Sense Investing