‘EXISTENCE … being … essence … nature … reality’ – these words appear on the first page of a notebook started by Peter Mark Roget in 1805. Son of a Swiss pastor, Roget studied medicine in Edinburgh and from 1805 onwards held a succession of medical posts in Manchester and London. Roget's father died when he was 4, and his mother and sister both suffered from bouts of severe depression. His mother's brother, the campaigning MP Sir Samuel Romilly, cut his throat after his wife's sudden demise and died in his nephew's arms. Fifteen years later Roget's wife Mary passed away at the age of 38. Fascinated by the shades of meaning expressed by synonyms, Roget kept his own depression at bay by jotting words down obsessively. Eventually in 1852 his list was published, and since then it has never been out of print. Roget's Thesaurus: the writer's best friend.