I don't believe that there's an event independent of all stimuli called "an original thought". Everyone is the sum of their experiences (and that's not even an original cliché), so any thought a person has is by definition derivative, even if it is only derived from personal experiences.
But you first need to define what a "thought" is. Do you mean an idea? Or a concept? If it's an idea, I firmly believe it is synthesised from your previous experiences. if it is a concept, that is a bunch of ideas arranged as a group, then it is a synthesis of your previous experiences and existing knowledge, or at least a selection of them.
And - assuming that you develop an idea which no one else has had, how do you know? It's new to you, but someone else may well have had the same one. Any number of concepts, seemingly original, have arisen independently of each other at about the same time. It happened a lot during the Renaissance, which at least appears to point towards it being a cultural thing.
The trick to "originality" is coming up with something {i]new to others, even though it may not be truly original. Hawking's ideas fall into that category. Most of what he says in simply a logical progression (to him) from some starting point based on his encyclopaedic knowledge of his specialist area. In other words, an "original thought" is often simply a new arrangement of or an extrapoloation of existing knowledge.
Well, that's my view!
