The term "narcotic" is believed to have been coined by the Greek physician Galen to refer to agents that numb or deaden, causing loss of feeling or paralysis. It is based on the Greek word "ναρκωσις" (narcosis)"," the term used by Hippocrates for the process of numbing or the numbed state. Galen listed mandrake root, altercus (eclata), seeds, and poppy juice (opium) as the chief examples. It originally referred to any substance that relieved pain, dulled the senses, or induced sleep. Now, the term is used in a number of ways. Some people might define narcotics as substances that bind at opioid receptors (cellular membrane proteins activated by substances like heroin or morphine), while others refer to any "illicit" substance as a narcotic. From a U.S. "legal" perspective, narcotics refer to opium, opium derivatives, and their semi-synthetic substitutes, though in U.S. law, due to its numbing properties, cocaine is also considered a narcotic.