Biographical articles on Gilbert Ryle—one of the 20th century’s greatest philosophers, remembered especially for his iconoclastic contributions to the philosophy of mind—tend to pass over his wartime years in silence, content with the meagre phrase ‘seconded to intelligence’. Yet those dark years were an important time in Ryle’s life and philosophical development. The investigation presented in this article extends from the time of his last philosophical publication before joining the army, in 1940, through to 1946, when his intelligence work continued alongside his Waynflete Professorship of Metaphysics. Ryle spent much of the Second World War spying on the enemy through a lens of broken German radio messages, including messages enciphered by the famous Enigma machine. He moved in the same shadowy ultra-secret milieu as some other notable academics, including Alan Turing, Gordon Welchman, J.L. Austin, Stuart Hampshire, and—on the other side of the Atlantic—W.V.O. Quine. Together with Hampshire, Ryle belonged to a small group of analysts described from on high in the British Secret Intelligence Service as ‘a team of a brilliance unparalleled anywhere in the Intelligence machine’. Ryle’s unit, the Radio Analysis Bureau (renamed later in the war as the Radio Intelligence Service) was part of the Radio Security Service (RSS), an organisation closely linked to Britain’s military codebreaking headquarters at the now famous Bletchley Park. Ryle’s principal work was analysis of the rich streams of intelligence traffic known as ISOS and ISK. Our account of Ryle’s war is based on letters, diaries, unpublished memoirs and, above all, top-secret documents by Ryle himself that remained classified until long after his death.