Who am I?

Who am I? How do I know what I do is not just a series of complex responses to stimuli? Descartes claims cognito ergo sum, but how do I know I actually think? The way I see it, thought is just opening of channels and passing synapse between cells. Heraclitus claims that nothing is, and everything is becoming, and I see value in that statement. Every moment in time at any part of the universe is never exactly the same. Likewise, the me in the past is not the same me of today. No one ever think the exact same thought twice, and scrutinize it enough, you are not the same you from a second ago. So who am I?

You might think I'm being nick picky about this sort of thing, and perhaps you are right. But consider this, when a person survives a serious trauma to the brain, their personality might so drastically that they act as if they're someone completely different. Yet from appearance, we say that's the same person. To me, this suggests that we have no control of who we are and what kind of person we want to be. I am nothing but a complex robot responding to a series of stimuli. Coursing though the process of nature, flowing through time like water flows down a river. There might be certain things I find painful or fear and try to avoid at all cost. But that's just certain chemicals being directed to certain positions in the brain by other factors. So why am I here? Actually, more fundamentally, is there even such a thing as an "I"?

I am not religious, so I cannot accept the idea that we have souls, and they go on while our body decomposes, although I wish that was the case. If we have souls, why doesn't anyone try to contact us from beyond the grave? With that many souls in the land of the not living, there must be someone persistant enough to find a way to get through, but there isn't. So I am forced to conclude that I am but a pint of water in the river of time. I am bothered by the insignificance and meaninglessness that is my undefined existance.