Warren Buffett, his mentor Benjamin Graham, and multiple other famous investors say that the key to successful investment is not risking one's principal.
Graham wrote in The Intelligent Investor:
"In the old legend the wise men finally boiled down the history of mortal affairs into the single phrase, "This too will pass." Confronted with a like challenge to distill the secret of sound investment into three words, we venture the motto - Margin of Safety."
Warren Buffett — Graham's most famous pupil — simply states the same rule as:
"Rule #1: Never lose money. Rule #2: Never forget rule #1."
Billionaire Value Investor and hedge fund manager, Seth Klarman, too says:
"Loss avoidance must be the cornerstone of your investment philosophy."
Peter Thiel, co-founder of PayPal and the first investor in Facebook, writes in his book Zero to One:
“First, only invest in companies that have the potential to return the value of the entire fund. This is a scary rule, because it eliminates the vast majority of possible investments. (Even quite successful companies usually succeed on a more humble scale.) This leads to rule number two: because rule number one is so restrictive, there can’t be any other rules.”
Do note that Thiel was referring to the highly focused nature of Venture Capital funds here. Thiel talks about only picking companies that can each repay the entire fund, and is more related to the counterintuitive Value Investing principles of diversification.
However, on a broader level, what Thiel is saying is also an application of the Margin of Safety; and possibly a good explanation of Buffett's own interpretation of the principle.
Buffett's and Thiel's statements are very similar in structure.
When we study the methods of most successful investors, we see similar principles expressed and applied in different ways. But while almost everyone has heard of the Margin of Safety, in one form or another, only a select few such as those above understand it and apply it successfully.
Graham gave the following definition for an Investment in The Intelligent Investor:
"An investment operation is one which, upon thorough analysis promises safety of principal and an adequate return. Operations not meeting these requirements are speculative."
The application of the Margin of Safety is actually best explained in Graham's definition of investment above, i.e., never risk the principal amount; and is also largely a function of the price paid.
The Margin of Safety is somewhat similar to what is often referred to in common financial parlance, especially in the context of Fundamental Analysis, as Upside; and is measured on GrahamValue by Intrinsic Value(%).
But do note that the actual Upside value for an investment would normally be higher than its Margin of Safety; since Upside also has to consider any potential future increase in Intrinsic Value of the investment, resulting from retained Earnings and the ensuing growth in Per Share Earnings and Asset Values.
The Margin of Safety only considers current stock prices and Intrinsic Values, and can be defined as:
"A favorable difference between price on the one hand and indicated or appraised value on the other. That difference is the safety margin."
Graham repeatedly emphasized this counterproductive nature of risk in investment. Again, from The Intelligent Investor:
"There has developed the general notion that the rate of return which the investor should aim for is more or less proportionate to the degree of risk he is ready to run. Our view is different. The rate of return sought should be dependent, rather, on the amount of intelligent effort the investor is willing and able to bring to bear on his task."
So Buffett was being quite literal when he said "Rule #1: Never lose money. Rule #2: Never forget rule #1.".
The following clip of the Buffett Rules is from Adam Smith’s Money World: How to Pick Stocks & Get Rich that aired in 1985 on PBS. This is also possibly Buffett's first major television appearance; and he explains both the concept of the Margin of Safety, as well as the importance of thinking independently.