Galen devoted to a polemic against Favorinus in "De optima doctrina", opposing Favorinus’ thesis that the best instruction consists in the argument in which one speaks, in each particular question, in favour of opposite sides. Galen's treatise says that Favorinus wrote a work "On the Academic Disposition" also called "Plutarch" and a work against Epictetus named "Against Epictetus" staging one of Plutarch’s slaves, Onesimus, arguing with Epictetus. Favorinus wrote "On the Kataleptic Fantasy" in which he is said to have denied the possibility of katalepsis, the key notion of Stoic epistemology.