The ongoing interpretation of Dupuit's conflict with the French liberal school on the question of justice consists of a simple opposition between public utility and natural rights. This paper aims to show that Dupuit's position is far more complex: justice is based on welfare - and thus on public utility - although natural rights are not excluded from his scheme. An understanding of Dupuit's concept requires a clarification of the differences between three notions - public interest, welfare and public utility - and, by the same token, of the factors linking each notion to the others.