In recent years, measuring devices and movements, such as the social investment perspective, have gained traction, allowing organisations to show the impact and social value of their work. A common characteristic is the persuasive rhetoric of how evaluation, quantification and measurement can improve human service organisations. The aim of this article is to unfold the argumentation for social investment fund (SIF) evaluation in human service organisations in three Swedish municipalities, exposing the logics according to which such proceedings are justified and legitimised in contemporary Swedish welfare state. The article draws on qualitative data from interviews with managers, evaluators and social investment staff, and evaluation reports from three Swedish municipalities. Using modality theory, according to which argumentation and reason are performed, findings reveal that SIF evaluation is promoted as a scientific construct with the possibility of producing certainty in the form of evidence regarding what works in a linear knowledge transfer model. Also, municipalities are portrayed as morally responsible for conducting SIF evaluations, emphasising that despite methodological challenges, pursuing such evaluations remains worthwhile. In this sense, SIF evaluation promotes an argument centred on two mutually reinforcing themes: scientific legitimacy and societal legitimacy.