We know we have a body as us. But if we loose our hand for example, we are still ourselves. Same is true about other organs like ear, eye, leg etc. But if our brain gets transplanted to some other body, we change to a person in that body. That's why some believe we are our brains as ourselves, our brains as a whole. I am differing in this issue, because our brains can get damaged partly, and we still would be our selves. For example we may loose our sight from something hitting our head, and it would be just like loosing our eyes, and we are still ourselves. So we should be something inside our brains. Then the issue becomes how much inside of our brains are we? We are the center part of our brain or a neuron inside the center part, or a molecule inside that neuron, or an atom inside that molecule, or ... a fundamental particle that is next to nothing in there? That's what I believe what we are as our selves, a fundamental particle deep in the center of our brain.
After all fundamental particles are not breakable. There is no matter smaller than a fundamental particle. But fundamental particles can do things, at least and at most one thing as what our selves can do. At least one thing we are capable of doing as ourselves, because we have choice. And at most one thing we are capable of doing as ourselves, because we are unbreakable as a fundamental particle. If we would be able to do more than one thing, we would be breakable into smaller pieces that at the final point each are only capable of doing one thing. It is like the theory of machines, that a machine can be broken up into pieces that each has only one degree of freedom. Of course we are not machines, but as a fundamental particle we only can have one kind of deed as our fundamental choice, our true selves' only degree of freedom. Otherwise we are not yet a fundamental particle, and we can still be smaller.
I advise you not to rush to what deed that fundamental choice is. Because one's fundamental choice should be able to choose itself. Because if one's only capability is incapable of acting on itself, then that one is nothing. Again, if you are with me until now, I emphasize don't rush to what deed that fundamental choice is.
The fundamental choice, I choose for myself, and myself or somebody in my head asked me "is that true in itself?" and I happily found that true, meaning it can choose itself, was and still is "Giving Thanks". Because you see, if one is only capable of giving thanks, everything should be goods to her/him from God. And if everything is going to be goods to me from God, it is itself the biggest good from God, which of course I can give thanks for.
I chose "giving thanks" because of my beliefs and faith, and thanks to God that it was choosable by itself. If you followed me to this point, It was my responsibility to suggest you a fundamental choice that is choosable by itself, but if you don't like giving thanks as your fundamental choice, who knows there may be other choices that are choosable by themselves.
As the final remark, l can be broken up into even fundamental particles, it would be painful, but when all that is left of me is a fundamental particle, I feel like I am still able to start a being by myself, by giving thanks to capability of giving thanks, and get going from there. Yeah I am saying if nothing is left for me but myself, I can still have a life to live. But don't think I am after being all by myself. My life is giving thanks, and for that I should be thankful for all the goods there are. And I love giving thanks and living with everything in the universe, and giving thanks for them all is so much more lovely to me.