Solipsism

"You can search the skeptical literature all you want. You will never find anyone who has ever argued, for any X, "I haven't seen X, thus X cannot exist."

Go ahead and prove me wrong. Find a quotation that says anything like that, and post it here."

I dislike the word "prove" out of context with logic, science, or law almost as much as I dislike verbal absolutisms such as "never". But to answer you question, this view has a pedigree going back to the Greek presocratic sophist Gorgias (c. 483-375 BC) who is quoted as stated:

Nothing exists;
Even if something exists, nothing can be known about it; and
Even if something could be known about it, knowledge about it can't be communicated to others.

Thus, he might say, Churchill doesn't exist, but if he does exist, his existence cannot be communicated. This relates to epistemological solipsism, in which "only the directly accessible mental contents of the solipsistic philosopher can be known." I don't know how deep an answer you want from me. The tone of your post suggests that you don't want me to get into Humean causality or Kantian idealism but were trying to score a late night point.

http://en.wikipedia.org/wiki/Epistemological_solipsism