Father John A. O’Brien (1893–1980), longtime Catholic chaplain at the University of Illinois and influential priest-social scientist, engaged both mainline thinkers and other Catholics on key social questions of the interwar era by employing rhetoric heavily seeded with science. Partly hoping to demonstrate Catholic openness to modernity while parrying longstanding accusations of Church obscurantism, O’Brien was one of a small, yet influential, cadre of progressive Catholics who wrote on topics straddling public ethics and public policy by using ground rules favored by non-Catholic, even non-religious, interlocutors. O’Brien thereby sought a via media between the Church and thought currents of the age. In the 1930s, his works on contraception emphasized the newly-configured “rhythm method” as a scientifically infallible way for married couples to separate sex from procreation in good conscience. The manner in which O’Brien argued in favor of rhythm—and the ends he emphasized—may have unintentionally contributed to a broader shift in the public sphere’s framing of the contraception issue. Consequently, contraception became primarily a technical question for scientists to deal with, a question that increasingly did not concern putatively obscurantist churches.