In this article I provide a conceptual analysis of an underexplored issue in the debate about effective altruism: its theory of effectiveness. First, I distinguish effectiveness from efficiency and claim that effective altruism understands effectiveness through the lens of efficiency. Then, I discuss the limitations of this approach in particular with respect to the charge that it is incapable of supporting structural change. Finally, I propose an expansion of the notion of effectiveness of effective altruism by referring to the debate (...) in political philosophy about realism and the practical challenge of normative theories. I argue that effective altruism, both as a social movement and as a conceptual paradigm, would benefit from clarifying its ideal, taking into account the role of institutions, and expanding its idea of feasibility. (shrink)