The work of many scholars over the course of more than a century has uncovered the major sources for almost the whole of Sir Thomas Malory's Morte Darthur. It has also revealed the cumulative importance of Malory's use of minor sources, which adds an unusual wealth of detail to Malory's story. Understanding how Malory combined major and minor sources clarifies the creative method of a significant medieval author, which is why academic criticism of the Morte Darthur routinely takes Malory's sources into account.

The only section for which no major source survives is ‘The Tale of Sir Gareth of Orkney’, and so scholarly scrutiny has tried to determine whether it too was derived from a source that is now lost and, if so, what might be deduced about it. Most of the published scholarly argument has been in favour of a lost source that resembled to some extent the romances of the Fair Unknown story type, but the contrary argument, that Malory constructed this tale himself, has also been proposed.

Given the hypothetical nature of all lost sources, the argument that Malory built this tale himself may seem to be the simpler hypothesis and therefore the one to be preferred. However, such a conclusion is inherently unlikely, as it would require Malory to go against his own wellestablished pattern of composition as shown by the rest of the Morte Darthur. Also, even if one were to reject a single lost source, ‘The Tale of Gareth’ still has demonstrable relationships to the Fair Unknown romances, and Malory would then have had to have patterned ‘The Tale of Gareth’ after them. Thus the tale would still have its sources. Further, the hypothesis that Malory constructed ‘The Tale of Gareth’ from multiple sources rather than a single major lost source is not easily able to explain the elements in this tale that correspond to Fair Unknown romances that Malory would have been unlikely to know, such as the German Wigalois and the Italian Carduino.

No argument that relies on a hypothetical lost source can ever be said to be settled beyond all doubt, but in this case the lost source hypothesis has a greater explanatory power and requires fewer unverifiable assumptions that any other yet proposed. Therefore, until it should be refuted by a detailed argument, it will be a safer conclusion than the alternative, which is itself merely a competing hypothesis.