The opening ceremony of the London 2012 Olympics would not have been complete without a performance of “Jerusalem”. The solo voice of 11-year-old Humphrey Keeper soaring over the stadium’s reconstruction of “England’s green and pleasant land” struck that special chord of mystical patriotism that only the poetry of William Blake can touch.To many of his contemporaries, Blake (1757-1827) was mad. The word crops up again and again in contemporary references, often in conjunction with the word “genius”. “I suppose this man must be mad, but he draws very well,” was the response of the antiquary Francis Douce to his first encounter with Blake’s imagery in 1794. Samuel Taylor Coleridge, after reading Songs of Innocence and of Expe