To a common view of mythical and autistic economics, reduced to its purely academic version, and disconnected from reality, we oppose an alternative vision of economics as an engineering, acting on this reality and transforming it to be like theory. We develop examples from finance, econometrics, or climate change, and we mobilize concepts from the history and the sociology of science, specifically in the tradition of science studies, as those of performativity (Callon) and regimes of knowledge and expertise (Pestre), which allow us to describe different forms of intervention by the economist in the city.