He later said that he had come with the passage of time to feel that he had failed earlier to appreciate some of the important things that Keynes was trying to say. Keynes, in turn, was very critical of Pigou, mentioning Pigou at least 17 times in "The General Theory of Employment, Interest and Money", usually disparagingly. Keynes states that Pigou is unable to devise any satisfactory formula to evaluate new equipment against old when, owing to changes in technique, the two are not identical. I believe that the concept at which Professor Pigou is aiming is the right and appropriate concept for economic analysis. But, until a satisfactory system of units has been adopted, its precise definition is an impossible task. Pigou had strong principles, and these gave him some problems in World War I. He was a conscientious objector to military service when it required an obligation to destroy human life. He remained at Cambridge, but during the vacations was an ambulance driver at the front for the Friends' Ambulance Unit, and insisted on undertaking jobs of particular danger. Towards the end of the war he reluctantly accepted a post in the Board of Trade, but showed little aptitude for the work. He was a reluctant member of the Cunliffe Committee on the Currency and Foreign Exchange (1918–1919), the Royal Commission on the Income Tax (1919–1920), and the Chamberlain Committee on the Currency and Bank of England Note Issues (1924–1925). The report of the last body was the prelude to the much criticised restoration of the gold standard at the old parity of exchange. Pigou was elected to the British Academy in 1925, but resigned in 1947. In later years he withdrew from national affairs and devoted himself to more academic economics and writing weighty letters to "The Times" on problems of the day. He was a foreign honorary member of the American Academy of Arts and Sciences, a foreign member of the Accademia dei Lincei, and an honorary resident of the International Economic Committee. He loved mountains and climbing, and introduced climbing to many friends, such as Wilfrid Noyce and others, who became far greater climbers. An illness affecting his heart developed in the early 1930s, however, and this affected his vigour, curtailing his climbing and leaving him with phases of debility for the rest of his life. Pigou gave up his professor's chair in 1943, but remained a Fellow of King's College until his death. In his later years he gradually became more of a recluse, emerging occasionally from his rooms to give lectures or to take a walk.