“Many of those who face adversity like this at 89 choose to hide it,” he wrote. “My life has always been an open book. Some chapters of my life have been great. Others not so much.”Pickens built one of the country’s biggest independent oil companies, Mesa Petroleum, before moving on to BP Capital and founding clean transportation fuels company Clean Energy.He rose to national prominence in the 1980s when he led a series of takeover attempts, targeting Phillips Petroleum, Unocal and Gulf Oil, among other companies.He’s also given away more than $1 billion to philanthropic causes, according to BP Capital’s websiteThe fall, he says, is just a set-back. Pickens said he can still comprehend and process information well, but has difficulty speaking clearly. He remains upbeat about the future.“That’s the way to approach life. Be the eternal optimist who is excited to see what the next decade will bring,” he said. FILE – In this Oct. 8, 2015 file photo, T. Boone Pickens appears on the “The Intelligence Report with Trish Regan” program, on the Fox Business Network, in New York. Pickens, who recently suffered from a series of strokes, took a “Texas-sized fall” that put him in the hospital last week. In a posting to LinkedIn, the corporate raider and legendary oilfield wildcatter said that he is still mentally strong but that as far as his life goes, “I clearly am in the fourth quarter.” (AP Photo/Richard Drew) The health of T. Boone Pickens, the famed 89-year-old oilfield wildcatter, financier and corporate raider, has taken a turn for the worse after a “Texas-sized fall” put him in the hospital last week.In a posting to LinkedIn, Pickens said that he is still mentally strong but that as far as his life goes, “I clearly am in the fourth quarter.”Pickens, the founder and chairman of the Dallas investment firm BP Capital, suffered several strokes over the holiday and has been undergoing speech therapy. He said that he’s regained 90 percent of his speech through aggressive therapy and determination.