"To each according to his ability, to each ability according to his works" constitutes the founding slogan of the Saint-Simonian doctrine (1825-1832). A century and a half would pass before Sen and Nussbaum developed their capability approaches, designed to consider issues of human development and quality of life. Given the prominence of capability approaches in the context of modern theories of justice, and perhaps also due to the natural analogy between the words 'capacité', 'ability', and 'capability', there is a clear tendency in the literature to analyse the Saint-Simonians' contributions to justice based on the assumption that there is a conceptual link between the terms capability and ability. This paper claims, however, that the elision of these terms is unjustified, and is a source more of confusion than of enlightenment. A capability is an evaluative space for justice, while an ability is a property of individuals. The former is defined essentially in the domain of consumption and individual accomplishment, while the latter is clearly seen as a contribution to the theory of efficient production. Finally, these differences reveal a contrast in the focus values: the ability approach insists on efficiency, while the capability approach focuses on agency.