Book excerpt: After a series of adventurous jobs around the world, Sam Sheridan found himself in Australia, cash-rich and with time on his hands to spend it. It occurred to him that he could finally explore a long-held obsession: fighting. Within a year, he was in Bangkok training with Thailand's greatest kickboxing champion and stepping through the ropes for his first professional bout. But one fight wasn't enough, and Sheridan set out to test himself on an epic journey into how and why we fight, facing Olympic boxers, Brazilian jiu-jitsu stars, and Ultimate Fighting champions.