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.
"Given the performance of certain stocks, we wonder if the market has adopted an alternative paradigm for calculating equity value," he wrote. "What if equity value has nothing to do with current or future profits and instead is derived from a company's ability to be disruptive, to provide social change, or to advance new beneficial technologies, even when doing so results in current and future economic loss?"
Tesla and Amazon shares are up 58 percent and 29 percent this year, respectively, compared with the S&P 500's 15 percent return through Monday.
"Our view is that just because Amazon can disrupt somebody else's profit stream, it doesn't mean that Amazon earns that profit stream. For the moment, the market doesn't agree," he wrote.
Einhorn was also not satisfied with the modest decline in Tesla's stock after the company's recent financial results.
"Tesla had an awful quarter both in its current results and future prospects. In response, its shares fell almost 6%," he wrote. "We believe it deserved much worse."
The investor doesn't know when value investing will work again.
"The knee-jerk instinct is to respond that when a proven strategy is so exceedingly out of favor that its viability is questioned, the cycle must be about to turn around. Unfortunately, we lack such clarity. After years of running into the wind, we are left with no sense stronger than, 'it will turn when it turns.'"
Einhorn's Greenlight Capital returned 6.2 percent in the third quarter, bringing its performance for 2017 through September to 3.3 percent, according to the investor letter. In comparison the rose 14.2 percent in the same year-to-date time period, according to the investor letter.
Greenlight, Amazon and Tesla did not immediately respond to requests for comment.
— CNBC's Leslie Picker contributed to this story.