Something I run into relatively regularly is the idea that people in the past were stupider than modern people. (Granted, it's not usually stated as bluntly as it was today, when a student explained something we were talking about by saying literally that medieval people were stupider than modern people, but the idea frequently underlies other comments.) I'm curious about how students define "the past" and "modern" in thinking about this - I suspect that they actually draw (entirely unconsciously) on an old school, secular humanist Enlightenment vision of history that disses the Middle Ages, and that they don't actually believe that the ancient Greeks and Romans were less intelligent than people today - but I'm sure that this idea pops up in many fields of history. Apart from the Enlightenment vision, I think much of it derives from an idea of progress - that past history is a linear progression to the present and that progress entails improvement: hence, people today must be smarter than those in the past.
This is one of those fallacies that I, and many instructors I know, have railed against many times. But I feel a little unfair in doing so, because I can actually pinpoint the moment when, viscerally, I came to understand what I now tell my students. I'm sure that before that time I didn't blurt out in class that people in the past were stupider than those today. But somewhere I absorbed the idea that because medieval sources didn't try to tell us modern people what we wanted to know, they were deficient. They didn't write "real" history.
Well, duh - of course they didn't write "real" history. They didn't know what it was. Not because they were stupid, but because the concept didn't exist. They didn't write for the same reasons we do; they had their own reasons. History meant something else entirely.
My realization came, embarrassingly late, in grad school, when I read Walter Goffart's The Narrators of Barbarian History. I think this was really the first time that I saw a scholar say, explicitly, bluntly, that because we find particular medieval authors hard to understand and that because they don't write what we would write, this does NOT mean that the texts are corrupt or deficient or inferior. It was one of those great DUH! moments. Clearly, if Goffart needed to say this so clearly, I wasn't the only one who felt that way (and I suspect that those studying the early Middle Ages have to combat such assumptions even more than I do, given how many of their texts only survive in later copies, sometimes copies of copies of copies, and that it's easy to explain difficulty away by saying that the text is corrupt). Nonetheless, his book illuminated for me some of the assumptions lurking in unexamined corners of my brain.
In my defense, I'll point out that when writing my senior thesis in college, I read an AWFUL lot of nineteenth-century editions of medieval sources, editions produced by erudite and eccentric independently wealthy gentlemen who wanted little more than to be the knights and nobility whose words they inscribed. (In one of my favorite editions, the editor deplored the need to have issued a revised edition, but there had been so many errors in the first edition that it had really had to be done. It was too bad, too, he went on, because the organization sponsoring the edition had provided a really bright undergraduate [this would have been an Oxbridge student, mid-to-late nineteenth century] to aid with the editing. When they asked the original editor why there were so many errors, he said that he had been so impressed by the undergraduate's work on the first two pages that he didn't bother to check the rest.) Whatever their reverence for the social structures and material records of the past, these gentlemen had no qualms about decrying the inadequacies of the authors of those records. (Another favorite? The cataloger from the British Library who described one of my treasured texts as, essentially, so much papist claptrap. I don't remember the exact words, but "papist" was definitely one of them.)
This brings me to one of my cherished theories about studying the Middle Ages: that we're not even figuring out what happened then, as much as we are trying to dismantle assumptions about the Middle Ages that the Victorians constructed and then bequeathed to us. (This has been especially true, I think, for medieval women's history.)
In any case. I'm not going to stop challenging my students when they tell me that medieval people were dumb. But I'm also going to remember that it's really hard to escape the mental structures that support your world so strongly that you don't even realize they're there. And as a society, we Americans sure do love the narrative of self-improvement and the idea of progress.