One effect of working with original sources in mathematics classes that is repeatedly mentioned is the stimulation of reflection on mathematics and, as a result, a change in the students' mathematical beliefs. In the context of the teaching project presented here on working with Bernoulli's first textbook on calculus, this could be impressively confirmed empirically by means of a qualitative study. The article is dedicated to the conception of the teaching sequence, as well as first results of the empirical evaluation, and also poses the question of the conditions for dealing with original sources in a digital setting in a reflection-stimulating way.