Former Treasury Secretary Larry Summers spoke at the International Monetary Fund on Wednesday, warning against austerity measures amid a tepid economy. Yahoo Finance sat down with him to get little more color on the economy and to find out what keeps him up most at night. “I think the prospect of Donald Trump being President would be the gravest threat to our prosperity, our security, and our freedom in my adult lifetime,” Summers said. “That's the thing I would worry most about.”