stocks as pieces of businesses,serve you rather than to instruct you,margin of safety

Warren Buffett: The three most important lessons I learned were all from the same book, The Intelligent Investor. It was written first by (Benjamin) Graham in 1949. They appear in chapters 8 and chapters 20.

The first is, to look at stocks as pieces of businesses, not as little items on a chart that move around, not as ticker symbols, not as something that might split next week or next month or something of the sort. But, rather, to look at the business, value the business, divide by the shares outstanding, and decide whether you really want to own a piece of that business at that price.
The second one was his commentary about your attitude toward the stock market. That it is there to serve you rather than to instruct you, and he used the famous Mr. Market example of that. That attitude is fundamental to making money in stocks over time.
And the final item he talked about was margin of safety. When you buy a stock that you think is worth 10 dollars, you don't pay $9.95 for it, because you can't be that precise in estimating its value. So you leave a considerable margin of safety for both what you don't understand and for the vagueries of the future.
And those three ideas, which I learned when I was 19 years old, have been the bedrock of everything I've done since.

His online bridge name is T-Bone, and he lists his age as 103 so that people will give him the benefit of the doubt if he does something "dumb."