From Thomas De Quincy via Coleridge to Berlioz, a second-generation opium addict, Daisy Hay and Richard Davenport-Hines discuss why drugs were thought integral to creativity first in England and later in France. They tell Matthew Sweet and an audience at Imperial College London about opium as pain relief and creator of dreams and constipation, why arsenic was the Viagra of its day, and why it's just possible that Paris was as revolutionary as it was in the 19th century because it was full of drug-taking rebels.