I would go back in time to visit some medieval people who would very kindly explain to me what the texts I am reading mean. Then I would footnote all of that in my thesis as "personal communication" and answer all my viva questions with "Yeah? I think you'll find that I actually asked him and he said I was right. Oh, and by the way, turns out King Arthur and Robin Hood were both real and as cool as the films make out - in fact, cooler."