It is not possible for anyone to have an impossible thought. Every thought that you or anyone else can imagine is real. That being said I can think of the idea of impossibilities therefore I know that it also must be real. This means that there is matter that was once in our universe that is now gone. Impossible is a term used to describe matter that is no longer part of our universe. We cannot think of these impossible things because we are no longer comprised of the matter required to do so.