@levi "I don't believe, what you say" is already a way to say "I don't believe, what you say". ? It's synonym to "You haven't convinced me (yet)". A statement like this isn't that significant. You might call it selfe-evident, since it comes from the best informed source thinkable and the source is unlikely to err on that point - might lie, but that's hard to determine and isn't very important to the argument. "What you described, is impossible" is another beast entirely - wouldn't you say?