"Of all the things humans rely on plants for--sustenance, beauty, fragrance, flavor, fiber--surely the most curious is our use of them is to change consciousness: to stimulate or calm, fiddle with or completely alter, the qualities of our mental experience. Take coffee and tea: people around the world rely on caffeine to sharpen their minds. We don't usually think of caffeine as a drug, or our daily use as an addiction, because it is legal and socially...