Galen. On my own opinions. Galeni De propriis placitis. Edition, introduction and translation by Vivian Nutton. CMG 5.3.2.

De propriis placitis is Galen's final work in which he reflected on some of the fundamental medical problems that occupied him throughout his long career. "This treatise is not quite the comprehensive survey of a lifetime's work that the title might lead one to expect; in fact, it is surprisingly disorganized and hardly useful as a clinical guide. It is rather more accurately seen as a work about epistemology and medical methodology, in which Galen considers what aspects of medicine are knowable or not and how one formulates principles in the face of our inconsistent knowledge of the body and, indeed, of the world" (http://repository.upenn.edu/cgi/viewcontent.cgi?article=1018&context=classics_papers, accessed 02-2017).