When I was researching my dissertation, I read through every medieval writing that mentioned the grail or any aspect of the grail legend. I was amazed at how entirely inconsistent the entire body of work was. The first continental writer to treat the subject, Chrétien de Troyes, described it as a platter. Wolfram von Eschenbach likened it to a small black stone. The Welsh called it a dysgyl, which could be any number of items. And that was all within the first century of extant grail stories.
The inconsistency confused me at first. The most reasonable explanation was that the grail was a creation of the medieval romance writers who then felt free to interpret it however they pleased. But everything else in the romances is so distinctly Celtic. Besides, dozens of scholars over more than a century have tackled the problem and treated it as though it was not a creation but a part of a tradition extending back well before the first extant story.
So I took another look. As I read through the works of various scholars, I realized two things that would not occur automatically to the modern person; that interpretation of a preexisting story was an author’s only avenue of creativity and that each author was patronized by a wealthy and powerful man who insisted on adding specific details to the story.
Interpretation, or as Chrétien called it, sens, is a much different approach than the modern perspective of creativity. Where we only stay within the confines of the barriers we set for ourselves, a medieval author could not create anything. To write a story meant only to have rewritten a previous one. An author could change details, reinterpret themes and details, but the story itself had to remain essentially the same. In other words, the grail could not have been created out of nothing. It had to come from source or other inspiration.
Patrons could be much more demanding on stories than the creative needs of an author. In my studies of the Arthurian materials, I have seen them influence geography and cause the insertion of contemporary persons. They have insisted that entire plots be rewritten and themes be added that overwhelm the entire story. On a couple occasions, entire endings have been erased at their command. It should come as no surprise, then, that the most popular topic of early Arthurian literature had been given a plethora of interpretations.
I did not base this conclusion solely on what I found in the grail stories, or even in the Arthurian works. Interpretation was a universal approach to all aspects of writing in the Middle Ages. The great thinkers of the period wrote treatises explaining and reinterpreting philosophers of the past. Chaucer collected legends and wove them into a larger story. In that respect, the grail legend is a part of the period in which it was written.
This means that to understand the true nature of the grail one must first account for all the creative, political, and personal motivations of all the authors and patrons associated with the grail legend. That was a massive undertaking.