Daniel P. B. Smith wrote:
I don't see what that can't be broadened just a bit. For example, let's suppose a library has an online catalog... let's say an online catalog that's accessible to anyone. (Two that come to mind are the Cornell University Library, and the 16,000-volume public library of Bergen-op-Zoom in the Netherlands... well actually it seems to be offline but it was available a few years ago).
You can't prove a negative, but you can certainly say "his book is not in the Cornell University Library" or whatever, and cite a link to the search or a description of how to do the search. This doesn't seem very different to me from a citation.
More precisely you can say that you could not find the book listed in the Cornell University Library Catalog. It's not the same even though the correlation between the two statements will be strong.
Ec