Warren Buffett spent much of his teenage years reading more about stocks and investment books. In 1949, his life changed when he got his hands on Benjamin Graham's new book, "The Intelligent Investor."

Graham began developing this system in 1928 with Columbia Business School professor David Dodd, and it serves as a guide to "value investing." The system advocates investing in stocks that trade below their value to guard against unpredictable future developments.

"The Intelligent Investor" is a simplified version of investment philosophy. For Buffett, "it was almost as if he had found God," his then-roommate Truman Wood told Alice Schroeder in her book "The Snowball: Warren Buffett and the Business of Life."

Buffett was admitted to Columbia Business School, and in the spring of 1951, he had the privilege of attending Graham's course.

He told Schroeder that he learned three main principles from it, which laid the foundation for his investment approach:

1. "A stock is the right to own a little piece of a business." ("Stocks represent ownership in a fragment of a company.")

Buffett said that people can easily get caught up in the volatile performance of stocks and forget that they represent ownership in a company. In the long run, the value of a stock is only as good as the company it represents.

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"The value of a stock is equivalent to only a fraction of what you would be willing to pay for the entire business."

2. "Use a margin of safety." ("Employ a margin of safety.")

Buffett emphasized the importance of buying stocks at a price significantly below their calculated intrinsic value to protect against the possibility of estimation errors or unforeseen events that might cause the value of the business to decline.The fundamental aspect of value investing is to take advantage of the margin of safety, which means that when the market price is significantly lower than its intrinsic value, one can purchase securities.

Determining a company's "true" value (its intrinsic value) is highly subjective.

Each investor has different methods for calculating intrinsic value, which may be correct or incorrect. Moreover, it is well known that predicting a company's earnings is difficult.

Therefore, using a larger margin of safety (for example, buying a stock valued at $10 for $7.50) can allow you to make substantial profits, and the market raising its price to fair value, while also providing protection during business stagnation.

3. "Mr. Market is your servant, not your master." ("The market is like a moody and emotional character, 'Mr. Market,' who is your servant, not your master.")

Graham personified the stock market with the character "Mr. Market," who is capricious and easily excited.

He asked his students to imagine Mr. Market as their business partner, offering them daily opportunities to buy and sell stocks.

The prices he offers are ultimately unreasonable, based on emotions.

However, he occasionally gives you the chance to profit from buying low and selling high.

The lesson here is that you should avoid market hysteria and base your decisions on your own research and valuation estimates."In another way, the market will fluctuate, sometimes even violently, but instead of worrying about the fluctuations, it is better to use them to your advantage by buying low in the market or selling when your holdings are overvalued" (provided that such stocks meet the criteria for value investing, not speculative stocks).