I don't think most of us know just how much and at the same time, how little, we know about music in the Middle Ages. For example, we have many manuscripts of early music, but it is not generally realized that up until the 12th century and beyond, almost all of them lack the name of the composer. Nearly all Gregorian chant, for example, is anonymous. The first important polyphonic repertoire was composed at the Cathedral of Notre Dame in Paris and the story of how we know who wrote at least some of it, is a musicological detective story. The cathedral was closely associated with the University of Paris, that was being created at about the same time. It is because of this association that scholars were able to learn something about the composers of the Notre Dame repertoire. The extant musical documents, three great books of music for services in not only Notre Dame, but all the churches of Western Christendom, were compiled a bit later, in the mid-to-late 13th century. The kind of music they contained was an enormous advance over the very simple part-writing previously as they contained music for three and even four independent voices. But who wrote this music? The people who copied and sang this music for generations did not know the authors' names as they were not given in the manuscripts.
There had been at the University of Paris a contingent of English theologians who went there for doctoral training. One of these, studying there around 1270 or 80, wrote a treatise called De mensuris et discantu (On Rhythmic Notation and Discant). We don't know his name either! One of the earliest scholars of Medieval music, Charles Coussemaker, published a collection of anonymous theoretical writings in 1864 and the fourth item was this treatise. Ever since, the author has been known as "Anonymous IV". The treatise begins with praise of "Leoninus magister" or Master Leonin who was said to be the best composer of organum (the term used for this kind of polyphony) and who made a great book of organa. That's all we are told. He also mentions someone named Perotin who was the best composer of discant and "better than Leonin." Then Anonymous IV lists Perotin's original works including Viderunt omnes fines terrae.
There is another small piece of evidence: in 1198 and 1199 the Bishop of Paris, Eudes de Sully, issued in letters, instructions about holiday celebrations in the cathedral. Keep the mummers and maskers out of the sanctuary and no processions of fools! Also, let there be good music and he names the feasts at which the music by Perotin listed by Anonymous IV would have been performed and even states the payment for organum quadruplum or four-part polyphony. We know it had to be by Perotin, because he is the only composer of such at this point in time.
The amazing thing is that the ONLY place where the names of the first two important composers in western music history are named is a passing mention in a treatise by an anonymous Englishman several decades later. Let's hear a little of this music, which still has an impressive effect to this day:
If you go to YouTube to watch/listen you will see that the date 1198 is cited. Now you know where that date came from: it is the first letter from the Bishop of Paris and of course is no indication of when the music might have been composed.