The U.S. stock selloff continues this week as investors question how much President Donald Trump’s proposed tariffs and other policies could impact the economy. Some investors have even questioned whether Trump’s moves could spiral the economy into a recession .

Even Trump admitted in an interview with Fox News on Sunday the economy was in a “period of transition” when asked if the U.S. was headed toward a recession.

“I hate to predict things like that, there is a period of transition,” Trump told Maria Bartiromo. “It takes a little time.”

Meanwhile, the S&P 500 Index, which tracks the performance of the 500 largest companies listed on the stock exchange, is down about 2% since the start of the week and almost 9% from its Feb. 19 peak. Even Goldman Sachs’ chief economist downgraded the entire U.S. economy this week.

But one of the most revered and storied investors in history has some sage advice for when markets are amiss.

“There is simply no telling how far stocks can fall in a short period,” Warren Buffett wrote in a 2017 letter to shareholders. But even when major stock drops happen, he suggested finding solace in Rudyard Kipling’s 1895 poem “If.”

“If you can keep your head when all about you are losing theirs… If you can wait and not be tired by waiting… If you can think—and not make thoughts your aim… If you can trust yourself when all men doubt you... Yours is the Earth and everything that’s in it.”

Buffett’s main point was to “keep your head,” or maintaining composure while markets move. In his letter, he was referring to major stock market declines, like during 2007–2009 when the S&P lost 50% of its value. Currently, the stock market isn’t close to such a severe decline, but the CBOE Volatility Index, which serves as Wall Street’s “ fear gauge ,” has spiked more than 80% since Trump took office.

How often do major stock selloffs happen?

While it can feel like a frenzy when it happens, stock market selloffs happen somewhat frequently. A market correction—defined as a decline of 10% or more from the most recent closing high—happens about every 1.8 years, according to stock market research company The Motley Fool . Since World War II, there have been 24 stock market corrections, data from Covenant Wealth Advisors shows.