On July 29, 1968, Pope Paul VI ended years of discussion and study by Catholic theologians and bishops by issuing an encyclical on human sexuality and birth control entitled Humanae Vitae. According to Boisi Center for Religion and American Public Life Director Mark Massa, S.J., the encyclical led to a paradigm shift in American Catholic thought that has had far-reaching repercussions. In his book The Structure of Theological Revolutions: How the Fight Over Birth Control Transformed American Catholicism (Oxford University Press, 2018), Fr. Massa argues that American Catholics did not simply ignore and dissent from the encyclical’s teachings on birth control, but that they also began to question the entire system of natural law theology that had undergirded Catholic thought since the days of Aquinas. He examines the work of theologians, including Monan Professor of Theology Lisa Sowle Cahill, concluding that theology does not develop one step at a time, in a linear fashion, but with large and even sudden shifts of meaning. Read a review of Fr. Massa’s book in the National Catholic Reporter.