Abstract This paper deals with the two notions of aition or aitia and prophasis in the Greek medical texts and asks the question of whether these words are synonymous or not. Therefore, it explores their different meanings in different contexts both in the Hippocratic and in the Galenic corpus. It also investigates how Galen understands these two notions when he reads them in the Hippocratic treatises and how he explains them in his commentaries to Hippocrates, and in particular, if he gives the same meaning to these words in his own treatises or if he uses them in a different way.