To win, the first thing you have to do is not lose.

Warren Buffett worked from the first principle he had learned from Graham:

To win, the first thing you have to do is not lose.

If one were to buy shares at depressed level, one is fairly confident that one will not lose, even if the loss is only on paper.  Buffett's principle rule for trading is this:

Never count on making a good sale, have the purchase price be so attractive that even a mediocre sale gives good results.