3 Investing Principles by Warren Buffet

• A stock is the right to own a little piece of a business. A stock is worth a certain fraction of what you would be willing to pay for the whole business
• Use a margin of safety. Investing is built on estimates and uncertainty. A wide margin of safety ensures that the effects of good decisions are not wiped out by errors. The way to advance, above all, is by not retreating.
• Mr. Market is your servant, not your master. Graham postulated a moody character called Mr. Market, who offers to buy and sell stocks every day, often at prices that don't make sense. Mr. Market's moods should not influence your view of price. However, from time to time he does offer the chance to buy low and sell high.

I think most investors forget about the last bullet, and that every investor, including myself should have a measure of what the right price of a certain stock must be.

Taken from The Snowball Warren Buffett and the Business of Life by Alice Schroeder