This paper draws on Hannah Arendt’s political thought to question the relationship between theory and practice in psychology and the public role of psychologists. One of Arendt’s main contributions to political theory was to underline the specificity of political action, and to stress that politics should not be ruled by pure theoretical reason or reduced to the technical management of social issues. Applied to psychology, a view of the relationship between theory and practice that ignores this specificity may well lead to efficacious applications, but it has a certain number of politically problematic consequences. These include the a priori disqualification of opinions, a loss of common sense, the a priori definition of the world as a set of variables, and the a priori definition of people as "material" to be shaped rather than as political actors. Such consequences are problematic insofar as they can lead to the exclusion of people from the public realm and undermine the very possibility of genuine political action. These points are illustrated and discussed through examples drawn from psychoanalysis and experimental social psychology.