It is quite shocking to realize that we're not the owners of the thoughts inside our mind, even the thoughts beginning with "I" ("I'm stupid", "Oh shit I will be late!", "I hate this fucking job!", etc).
We identify with these thoughts, believing we are their originator, but it's the biggest illusion of all.
Are you referring to bicameralism?
I'm only referring to my own experience.
See if you can stop thinking for the next sixty seconds. You can notice your breath, or listen to the birds, but do not let your attention be carried away by thought, any thought, even for an instant. Put down this book, and give it a try.
Some of you will be so distracted by thought as to imagine that you succeeded. In fact, beginning meditators often think that they are able to concentrate on a single object, such as the breath, for minutes at a time, only to report after days or weeks of intensive practice that their attention is now carried away by thought every few seconds. This is actually progress. It takes a certain degree of concentration to even notice how distracted you are. Even if your life depended on it, you could not spend a full minute free of thought.
The vast majority of us do not even know what we are thinking. I'm not sure there's much correlation with intelligence, either. If this sounds absurd to you, I can only recommend you try out meditation for a while until you see how disconcertingly true it is.
One day I hope the Economist publishes:
Can we know what we are thinking?
The inner lives of our species may be a lot richer than science once thought