“Io, io sono la tradizione! Io, io sono la chiesa!” Thus Pope Pius IX, asserting that he himself was the Church and its tradition, reportedly responded to Cardinal Filippo Maria Guidi on 18 June 1870. Their meeting took place only hours after what John O’Malley in his newest conciliar history calls “the best-known and most discussed incident” of the First Vatican Council (1869-1870). The attending bishops, sweltering in a Roman heatwave, had divided into two intransigent camps. A minority that opposed a definition of papal infallibility resist­ed an unpersuasive majority.