One of the things Nature can’t do, according to Galen’s excursus at the beginning of Book Three of De usu partium, is create centaurs. Galen’s arguments are actually very good, and he presents them in three neat steps in the rhetorical figure called concessio and/or, depending on your point of view, a perfect example of apagogical reasoning: centaurs couldn’t come into being; if they could come into being, it wouldn’t be possible to feed them, and even if all those obstacles could be overcome, the one advantage of increased speed on even ground would be dwarfed by a lot of disadvantages. So, aren’t you glad you’re not centaurs? But what makes this text particularly interesting is the fact that it seems to be the first extant Greek text which argues that all (not just some) myths are unreliable, complementing some passages of another famous treatise of Galen’s, De placitis Hippocratis et Platonis. But that is not all. Galen is just one step away from writing fantastic fiction – he does ask the “what if” questions which are a prerequisite for good fantastic literature. He seems to be the first who tried to imagine how it would feel like to be a centaur. He is also probably being intentionally funny: he is certainly imagining a series of ludicrous situations, and he does occasionally show a certain talent for comedy, e.g. in ch. 7 of De praecognitione.