Ramon Llull, sometimes anglicized as Raymond Lully, was a Catholic philosopher, theologian, poet, missionary, apologist and former knight, born in Palma de Majorca. He invented a philosophical system known as the Art, conceived as a type of universal logic to prove the truth of Christian doctrine to interlocutors of all faiths and nationalities. The Art consists of a set of general principles and combinatorial operations. It is illustrated with diagrams.