KEY POINTS
  • "Given the performance of certain stocks, we wonder if the market has adopted an alternative paradigm for calculating equity value," Einhorn writes in an investor letter to clients.
  • Greenlight Capital returned 6.2 percent in the third quarter, bringing its performance for 2017 through September to 3.3 percent.
  • In comparison, the S&P 500 rose 14.2 percent in the same year-to-date time period.

Hedge fund manager David Einhorn, who's become a billionaire by effectively identifying undervalued and overvalued stocks, is struggling right now and questioning whether the classic investing principles that made him will ever work again.

"The market remains very challenging for value investing strategies, as growth stocks have continued to outperform value stocks. The persistence of this dynamic leads to questions regarding whether value investing is a viable strategy," Einhorn wrote in an investor letter Tuesday obtained by CNBC.