From the perspective of others I am part of the world, but when I observe the world from my perspective I am nowhere to be found.
To observe is to create perspective. I can never cease to be the point of origin from my perspective. I observe that which is not myself.
I am most reminded of Hume and his discussion of the self being interpreted as a single entity, but not necessarily so in actuality. (we may actually be -- and from scientific evidence, most likely are -- a conglomeration of multiple fractured subconscious perspectives artificially combined to be perceived by the over-conscious as a single one.*) Hume also points out that the outside world as being nothing certain except perceived stimuli; we perceive an outside world, but our only access is through these stimuli and reactions being perceived. We do not actually have any kind of direct access to an external world, merely a conduit through which information flows. The information could, in fact come from anywhere -- our own imaginations, a computer program, a deity, a shared dream, a reliving of memories -- with just as much likelihood as what we claim is true -- an actual outside world.
Cogito ergo sum. I think, therefore I am. This is Descartes famous line.
"I think therefore you are" either means that the world is the only way we can validate our own existence. (after all, who would know they exist if they could not observe themselves, or be observed by the world, either literally, or else by interacting with something through touch, smell, sight, or sound. If we could not, we would only know... nothing. No senses, no time, no space, nothing....)
It could also mean that the world only exists in our consciousness; it is all a dream we are imagining and creating. The most common synonym to this is the Chinese saying: "But he didn't know if he was Zhuangzi who had dreamt he was a butterfly, or a butterfly dreaming he was Zhuangzi."
It is also true that you cannot observe the observer. Like a puppy chasing its tail, you can strain as much as you want, but despite twisting and turning around, you cannot see yourself either mentally or physically. Yes, you can see "yourself" in a mirror, but that is again, a false image. It is a shadow cast by light, and it is imperfect, both from the reflective nature (reversal) but also from the light traveling through distorting air, hitting the mirror with it's own blemishes and distortions, and then being received imperfectly and transformed into electric signals imperfectly and interpreted by a mind imperfectly. You are not actually seeing yourself after all. This is merely the visual physical side of things, it becomes even more complicated when you are looking deeper into what it means to "observer the observer." In any case, it cannot be done, if only for the logical impossibility of containing a complete visualization of yourself within yourself.
There is also the three "you's." The you that you think you are. The you that you want to be. And the you that you actually are. You will never truly know the last one, the closest you can get is by asking people who are honest that are close to you.