We study the influence of argument source on the evaluation of argument validity. We ask, whether partial, impartial and reluctant sources generate different evaluations of argument validity. Previous research has revealed that impartial and reluctant sources promote persuasion. We, however, look at evaluations of argument validity, rather than attitude change, because the assessment of argument quality is also an essential part of democratic discussion. We explore the source effect via a survey experiment where participants are asked to evaluate politically relevant arguments. To avoid the influence of participants’ personal opinions on the issues, participants are asked to take a general perspective when they evaluate arguments. Respondents are randomly allocated into four treatments: a Control treatment with no argument source; a Partial Source treatment, a Reluctant Source treatment and an Impartial Source treatment. Participants (n = 1600) are recruited from a pool of volunteers, and they form a representative sample of the Finnish population in terms of age, sex and residential area. The survey was conducted online in June 2018. Data is analysed mainly through OLS-regressions. Based on our results we can reject the null hypothesis of no impact of the argument source. Further, the impartial source gives rise to higher evaluations of argument quality compared to a reluctant or a partial source. We tentatively suggest that this influence goes through trustworthiness. We conclude that the argument source has an influence on the evaluations of argument validity, and that an impartial source generates highest evaluations of argument validity.