In my years as a student of Mary Morgan and later as her junior peer, I observed that one concept prompted her to react with caution and skepticism. That common notion was “influence.” In this chapter, I follow her cues to ask what are the legitimate grounds for claims of influence in historical explanation. Morgan’s writings have made us aware that the story of social science cannot be captured in simple reckonings of influence, and that long chains of actions are required to seat an idea in the mind, and longer still to set it to paper. My contribution to problematizing influence is to list the pitfalls of its uncritical use but also, once suitably redefined, its potential contribution to analysis. To illustrate my claims, I propose a test case, to study the “influence of Mary Morgan.”