A few of my readers have asked me about when the Middle Ages became the Middle Ages, and why they're called that in the first place. I get into some of this with my students, and am, in a few months, going to get into it even more, as I'll be co-teaching a new course on medievalisms (about which more later).
The short answer is that the nomenclature and the idea behind it are legacies of Renaissance humanism. In the late antique (late 3rd-7th centuries) and medieval periods, people who were thinking and writing about history, the world, and their place within it didn't think they were in the middle of anything. They thought they were at the end. St. Jerome, who lived and died in the Roman Empire, viewed history in terms of empires (Babylonian, Greek, Roman). St. Augustine viewed human history as comprising six different ages, and he was writing in the last age. Writing during some hairy threats to the Roman Empire around 410, he viewed the end of the Empire as a sign of the coming End Times. Peter Abelard referred to himself and his intellectual cohort as moderni, meaning that they were modern and of the moment. (Medieval is the old modern?)
It was Petrarch (d. 1374), a student of the classical Greco-Roman past, who wrote that he was living in an "age of darkness." No marble statuary could compete with the ancients, no one's Latin was a good as Cicero's, etc. Some of his Italian humanist successors looked to this view as a way of further explicating their own time and a way of distinguishing between themselves and the benighted ignorami who came before them. The "medium aevum," or "middle age" was in use as early as 1469 to describe the seemingly bleak period between the end of the Western Roman Empire and the rediscovery of what humanists considered to be the pinnacle of western achievement: the intellectual and artistic culture of the classical period.
But that term wasn't codified, at least in English, until the 17th century. In part, that's because there wasn't enough weight behind the term. It hadn't been around for long enough. Another reason is that the chattering, writing, thinking classes were still reading (and printing and translating and re-printing) books that had first been written in the Middle Ages, up through the middle of the 17th century. (For further reading, check out Alexander Murray's "Should The Middle Ages Be Abolished?" [sub. req'd.]) By the end of the 17th century, the "Middle Ages" had become an established period, and "medieval" began to refer to a set of practices, ideas, and viewpoints that were seen as largely incompatible with current, "modern" ways of doing things.
Still to come...musings on how we got from there to here: