The articles in this issue are a selection of the papers presented at the conference Knowledge as Assimilation, held at the University of Helsinki on 9-11 June 2017. The conference was the result of a collaboration between two research groups that have been established in Finland and Sweden from 2013 onwards: the research project Rationality in Perception: Transformations of Mind and Cognition 1250-1550, funded by the European Research Council (2015-2020) and hosted by the University of Helsinki, and the research programme Representation and Reality: Historical and Contemporary Perspectives on the Aristotelian Tradition, funded by the Riksbankens jubileumsfond (2013-2019) and located at the University of Gothenburg.