The Great Return
possible that some vague notion of the Graal Cup may have reached even Welsh country folks through Tennyson's "Idylls." But so far I see no reason to suppose that these people had ever heard of the portable altar (called Sapphirus in William of Malmesbury) or of its changing colours "that no man could discern."
And then there are the other questions of the distinction between hallucination and vision, of the average duration of one and the other, and of the possibility of collective hallucination. If a number of people all see (or think they see) the same appearances, can this be merely hallucination? I believe there is a leading case on the matter, which concerns a number of people seeing the same appearance on a church wall in Ireland; but there is, of course, this difficulty, that one may be hallucinated and communicate his impression to the others, telepathically.
But at the last, what do we know?
