This article establishes, on the one hand, that the use of the concept of capability as a simple “metric” of human development is a reductionist view of Sen’s intellectual enterprise. On the other hand, it aims to show that it is vain to expect from Sen a theory of justice in terms of rights to certain capabilities. We present two lines of arguments: 1) By taking the standard and original hypothesis of capability as the set of functionings that are feasible for a person to achieve (e.g., Sen 1987), we highlight four theoretical implications of Sen’s approach that the perspective of “formal welfarism” (Fleurbaey 2003; D’Aspremont 2011) does not allow us to understand; 2) We consolidate this reading by examining the hypothesis—under-explored until now—of capability as “effective power” to act in the direction of results that we value (Sen 2008, 2009). In both cases, we show that capability, for Sen, is anything but a “metric” of individual advantage and we confirm the idea that Sen is not a capability theorist like most commentators expect (Baujard and Gilardone 2017). Furthermore, the second hypothesis leads to incorporating from the outset the issue of moral obligation in the concept of capability. As a result, not only is capability not a metric of personal advantage, but it is not a representation of personal advantage at all. This is where we identify the genuine conceptual revolution of Sen which, contrary to what Ricœur (2004) thought, does not lie in the right-capability pair, but in the responsibility-capability pair, forcing us to rethink the standard framework of theories of justice. Finally, it appears that Sen shares the ideas of the proponents of the economics of the person (Ballet et al. 2014), the idea that a theory of justice in terms of rights to certain capabilities would remain trapped by a purely functional view of freedom. Above all, highlighting the responsibility-capability pair opens the way for new perspectives on people’s liberty and rationality, especially in questions of justice and collective choice.