This paper examines the methodological claim made famous by P. F. Strawson: that we understand what features are required for responsible agency by exploring our attitudes and practices of holding responsible. What is the presumed metaphysical connection between holding responsible and being fit to be held responsible that makes this claim credible? I propose a non-standard answer to this question, arguing for a view of responsible agency that is neither antirealist nor straightforwardly realist. It is instead “constructivist.” On the “Scaffolding View” I defend, reactive attitudes play an essential role in developing, supporting, and thereby maintaining the capacities that make for responsible agency. Although this view has relatively novel implications for a metaphysical understanding of capacities, its chief virtue, in contrast with more standard views, is in providing a plausible defense of why so-called “responsible agents” genuinely deserve to be treated as such.