I have already discussed this issue, of how difficult it is to prove that something isn’t the case, which is why the burden of proof has to be on the person making an extraordinary claim to prove that their claim is correct rather than the other way round.
Let me explain with an example. Suppose that I make a ridiculous claim. Here’s my claim. Enjoy!
I have a copy of an old book which I bought many years ago in a small bookseller’s in Albi called Recherche sur les Cultes et Monumens Celtiques d’Irlande, written by Alexis Bonjovi de Croquemonsieur, published in Montpellier in the year 1767. I believe this to be the only surviving copy. Based on manuscripts he had seen in an Irish chateau near Cognac, the author refers to the curious customs surrounding the entry of the ancient Irish chieftain to a room. Before he entered, his poet and his reacaire (or reciter) would precede him and the poet (making a gesture uncannily reminiscent of modern jazz-hands) would wave his hands on either side of his head and utter the word ‘Rásamatás!’ in a loud voice. The reacaire, standing directly behind the poet, would then cross his arms in front of his body with two fingers extended on each hand and utter the phrase ‘Ió dúd!’ The chieftain would then enter, with gold chains draped around his neck, to cries of ‘Bhásáááp!!!’ from the assembled company.
I believe these customs described by Bonjovi de Croquemonsieur to be the origin of many features of modern popular culture in America. Somehow they have survived the centuries, come into the Irish ghettoes and have found a new home among urban black populations where they are to be seen in modern jazz, rap and hip-hop with their razzamatazz, yo dude and whassuuup.
So, I make this absurd claim. But I don’t provide proof that the book exists or that it contains this story. I expect you as a sceptic to prove that this book doesn’t exist and that I didn’t make it up. Not very fair or reasonable, is it, to expect you to hunt through every library on earth demonstrating that this author and this book is a fiction? Surely the burden of proof should be on me to prove that ancient Irish poets invented jazz-hands? But there are many supporters of Cassidy who seem to be incapable of understanding this basic principle, that it is up to Cassidy or his supporters to prove that nonsense like uí bhfolaíocht án and sách úr exist and that sceptics like me shouldn’t have to prove that they don’t.