Nearly every culture throughout history has used chemicals that alter consciousness for spiritual exploration. In the 20th century these drugs caught the attention of scientists. Psychedelics, as they were named, proved effective at treating intractable illnesses like depression and addiction. And they became a tool for studying the mind, opening “the doors of perception,” as Aldous Huxley wrote. But those doors slammed shut when President Nixon declared psychedelics dangerous and medically useless.