Systematic errors (also known as biases) recur predictably in numerous particular circumstances, including academic research. Classical studies are not exempt from them, especially when scholars intent on interdisciplinary research but not sufficiently versed in disciplines other than their own „collaborate“ in circular reasoning. Retrospective diagnosis is a case in point. While the ‘communis opinio’ has accepted the diagnosis that Corellius Rufus suffered from gout, a close examination of Pliny’s Letter 1,12 using the results of social cognition theory will show that this actually begs the question.