In 1971 Terrence McKenna, a philosopher and ethnobotanist set out with his brother Dennis to travel the Amazon in search of drug-induced visionary experiences. That wild adventure led to a lifelong study of hallucinogens. Though Terrence died ten years ago, Dennis, now a botanist and lecturer at the University of Minnesota's Center for Spirituality and Healing, continues the quest. He tells Steve Paulson how shamanistic cultures use hallucinogens.