The quote from Val McDermid's "Northanger Abbey" highlights the challenge of proving a negative assertion. It emphasizes that certain claims cannot be conclusively verified or disproven, such as the existence of mythical creatures like vampires or personal identities. This draws attention to the philosophical debate surrounding the nature of evidence and belief.

In essence, the statement serves as a reminder that some assertions, no matter how peculiar, are inherently undemonstrable, which can complicate discussions around belief and truth. It underscores that without concrete evidence, we may find ourselves in a realm of uncertainty and speculation.