The way you've phrased your comments tells me you're a Value Investor: buy at a discount to true value. That's completely valid, but it's only one school of thought. The other is Trend Investing: buy at any price as long as the share price is going up. This is equally as valid as value investing.
In a loose sense, index funds can be seen as trend investing. 1) Buy stocks which are increasing in market cap 2) Drop stocks which fall below a market cap threshold 3) Value, such as P/E, doesn't matter
