In the 2000s, many European Union countries have established research impact assessment practices as part of the scrutiny of the third mission of universities. The Finnish research evaluation system has widely adopted a societal impact criterion. In this study, the question is what guiding principles does impact assessment rely on. This point of view is based on the experiences and opinions of experts interviewed for the study. Four implicit principles of impact assessment which guide assessment goals and practical implementations were found. The guiding principles have several social and methodological dilemmas due to liminal interpretations between social interests and academic endeavor. This study recommends that the evaluation scholars should consider the actual social purpose vis-a-vis consistent methodological approaches to assessment before applying 'all-round' solutions to scholarly fields.