Invention is rarely a component in the origin of oral traditions - and folklore entwined with literary and written works is clearly a good part of the story of how Arthur and his court evolved and came down through the centuries.

Question along this line have been asked so often, it was one of the first I answered when attending to matters on /r/AskHistorians over 9 years ago. The answer I developed subsequently appeared frequently; it became apparent that popular curiosity was fixated on this sort of thing, and so I incorporated it into the appendix of my Introduction to Folklore, which I developed for students while still teaching this subject. Here is an excerpt:

When I see the posts like this asking about whether there were real people or events behind legends, myths, and/or the ancient gods, I respond with several observations. First, the idea that the gods and heroes of legend are based on real people had an early proponent in the Greek, late-fourth-century BCE writer, Euhemerus, giving his name to this approach to myth and legend: Euhemerism. Folklorists generally regard the idea that there was an actual basis for most oral tradition as barking up the wrong tree, because the original “real” event behind a story is usually elusive and searching for that core is a futile exercise. In addition, research into how stories began usually concludes that they emerge in a rather spontaneous way, typically without an actual incident to inspire them.

A few examples: the Classical Greek story of Perseus is an early manifestation of a widespread folktale, catalogued by the twentieth-century folklorists Antti Aarne and Stith Thompson as AT 300, “The Dragon Slayer.” Was there a proto-Perseus who rescued a maiden from some sort of extraordinary threat or perhaps from some sort of human sacrifice? It is hard to answer that question, but it is not hard to imagine how far back in time that proto-incident would have had to occur: AT 300 is spread throughout Eurasia. It was collected from cultures that could have no conceivable literary connection with ancient Greece, and yet the shared assortment of motifs in the numerous variants clearly show some sort of genetic, that is, historically connected relationship. Would we need, therefore, to go back thousands of years before the first recordation of the Perseus story to find this proto-Perseus? It is much easier to understand that the folktale simply diffused and that one of its manifestations was in ancient Greece.

Now, let’s consider another example that has inspired a lot of spilled ink. A simple Google search for the “origins of King Arthur” provides more websites than one could easily read in a week. Was there a proto-Arthur? Perhaps. Maybe there were several. But what does that prove? Every society has remarkable characters, and it may be a natural process for these sorts of individuals to attract all manner of traditional stories that have nothing to do with the original inspiration of the cycle of legends.

So, what do we have with Arthur? Was there a core source (or sources) for this legendary character? Let’s concede for the sake of argument that the answer is yes. Now, did this individual have a great warrior at his side who became ensnared by the leader’s wife in the fashion of Lancelot and Guinevere? That is more problematic since this type of story is also associated with Diarmuid and Grainne in the Irish court of King Finn and with the Cornish stories of Tristan and Isolde in the court of King Mark. One could even argue that it is the story behind Helen of Troy. In fact, it appears that this was a widespread type of story that became associated with various courts of historical legend. We cannot conclude that every great king had a queen who was attracted to one of his warriors and coerced him to take her away. This is simply a story that was attached to cycles involving great courts. In short, the further one goes back to find the “real Arthur,” the less the candidate (or candidates) look like the King Arthur who has been beloved for centuries. The proto Arthurs are not really King Arthur. They may be seeds but they look nothing like the tree that would grow over the centuries. We do not hold an acorn and say “Ah, I have in my hand a mighty oak tree.” It is not yet a tree. It is a seed. And the two look very different even if they are genetically linked.

One more example: there is a widespread legend told by countless families of the ghostly appearance of a loved one in anticipation of news that the individual died. This became a popular tradition in post-Famine Ireland because so many relatives lived in North America or elsewhere. But it is frequently told by all sorts of people internationally. So, we can ask, are there real-life, actual inspirations for this legend? That is, do the spirits of the dead actually come to visit loved ones? Well, how the hell should I know? To paraphrase a famous line from the television show “Star Trek,” “Damn it Jim, I’m a folklorist, not a ghost hunter.” And I have no intention of becoming a ghost hunter. It doesn’t matter what is behind stories so much as it does that people tell these stories. I’m in it for that part of the game; I consider stories as they are told over time, to gain from that material some insight into the past, into culture, and into the human condition. I am a folklorist. And with that, my plate is full.

I’ve never seen any references to Arthur being inspired by William I, especially as the first mentions of Arthur appear centuries earlier and Arthur is a pretty passive king in the later romances and cycles of the 12th/13th centuries. In short, he doesn’t really do much. The stories are about his knights instead.

Lancelot makes his debut in Chretien de Troyes’ Arthurian Romances and he seems to be Chretien’s invention/popularisation. It’s through Lancelot that he explores the themes of courtly love, deriving from southern French culture which were brought north, in part, by the influence of Eleanor of Aquitaine. He doesn’t seem to tie in to other figures mentioned in earlier Arthurian legends from previous centuries, so in the case of Lancelot it doesn’t seem like there was a direct historical counterpart.

However we know that Arthurian tales predated being written down and had influence from Welsh/Celtic/Briton myth, and later writers put in their own contexts and spins, such as Geoffrey of Monmouth giving the Britons a Trojan origin story. So it’s perfectly possible and likely that some inspiration was taken from earlier folklore in later writings but direct conflation is very tricky to assess.