DIALOP's project of developing a "transversal social ethic" emerges in the context of contemporary socio-ecological crises and responds directly to Pope Francis' call for a "culture of encounter". But while the present conjuncture may open up distinctly new synergies between marxism/socialism and catholicism, it is important to draw on the lessons and intellectual resources of past attempts to bring these traditions into dialogue. This paper considers the encounter of Catholic social thought and socialism in the work of Mamadou Dia (1910-2009), Senegalese intellectual, politician and activist, who stands among the most significant West African theorists and actors of decolonisation. Having briefly served as the first PM of independent Senegal, Dia's arrest and imprisonment in 1962 is often seen as a path-defining moment, in which neo-colonial interests were safeguarded and alternative possibilities curtailed. Although a lifelong practicing Muslim and the author of several important works on Islam in African societies, Dia was heavily influenced by Catholic social thinkers, such as Teilhard de Chardin, and in particular the personalism of Emmanuel Mounier. This paper explores how he sought to synthesise these ideas with marxist/socialist concepts & anthropological insights, setting out a distinctive vision for "African Socialism" based on a "human economy" that overcomes social inequalities. It also discusses his attempt, in close collaboration with the Dominican priest and development planner L.J. Lebret, to bring these ideas into life through a national network of self-governing cooperatives & an innovative programme of "animation rurale" (participatory popular education). The chapter concludes by asking what we might learn today from Dia, not merely as an icon of the "road not taken", but as a thinker of the "transversal social ethic", whose work brings not only Catholic social thought & marxism/socialism, but also Islam & African tradition, dialogue.