i don't think there is a purely clear cut boundary at all. and one person that seems insane to someone can be perfectly sane to another. it seems to be a matter of perception.

i think when thoughts become uncontrollable in the sense that they are fully believed by the person without any reason, rationality or logic, then they are insane.

but then there are those who others think are insane, but have reason to be. like those that are considered insane because maybe they have been abducted by aliens [well they believe so]. we consider them to be crazy, but if they really had been abducted, would they really be crazy? no, i don't think so.