One thing alone remains unexplained, the mysterious Grail itself. Nor has any light been thrown from Celtic sources upon the incident of the hero's visit to the Castle of Talismans, his silence, and the ensuing misfortune which overtakes him. Where this incident does appear in a Celtic version, the Mabinogi, it is not brought in connection with the Grail, and it bears obvious traces of interpolation. The utmost we have been able to do is to reconstruct from scattered indications in different Celtic tales a sequence of incidents similar to that of the French romance. Let us, then, return to what may be called the central incident of the Grail legend in its older and purer form. And let us recall the fact that the hypothesis which finds a Christian origin for the whole legend has no explanation to offer of this incident. Birch-Hirschfeld can merely suggest that Perceval's question upon which all hinges is "eine harmlose Erfindung Borron's," a meaningless invention of Borron's. It is, indeed, his failure to account for such an essential element of the story that forms one of the strongest arguments against his hypothesis.
In the first place it must be noticed that the incident of a hero's visit to a magic castle, of his omission whilst there to do certain things, and of the loss or suffering thereby caused, occurs not once, but many times; not in one, but in many forms in the vast body of Grail romance, as is seen by the following list, which likewise comprises all the occasions on which one or other of the questers has come near to or succeeded in seeing the Grail:—
