The fact that there exists a word like "epiphany" (sp?) shows that thoughts can come to us as a result of something that trancends simple outside stimuli. Not sure about all of you, but i've had plenty of thoughts that came from nowhere.

Maybe it's a case of your mind just sort of going over collections of old info and coming up with new connections between ideas that didn't have any connection before. Maybe you just heard some new tid bit of info that makes a connection between two bigger bits of info and the connection just hits you one day.

Not sure if i'm even making alot of sense, but i believe that our thought process if far more complicated than a simple stimulus and response system.