My thoughts are unobservable, therefore I exist.

 It is  argued that it is the early realization of the fact that one’s thoughts are unobservable by others, one experience of the profound sense in which one is alone in one’s head.

It is perhaps most often vivid when one is alone and thinking, but it can be equally vivid in a room full of people. It connects with a feeling that nearly everyone has had intensely at some time the misplaced belief  that one’s body is just a vehicle or vessel for the mental thing that is what one really or most essentially is.

Source: Galen Strawson