When I use the word "self" in this article you may read it as personality or spirit or soul; the differences are not at all clear to me.
There is a common tale of a seer and a king. The seer tells the king there is no such thing as a chariot and offers to prove it if a chariot is brought before him. The seer removes the wheels and asks if they are a chariot; the king says no and the wheels are set aside. He continues removing parts and asking if each is a chariot, then setting them aside. Soon there is nothing left and the seer asks the king "Where was the chariot?"
I hope to avoid making this mistake when I talk about the self.
Nathaniel Branden pointed out that science can be divided into three broad categories with different levels of complexity. The first is physics. Most basic mechanisms are understood well enough to be accurately modeled and predicted. Experiments are repeatable. Working machines are built with great confidence. The second is biology. Some basic mechanisms are understood but many are not. Many experiments are impossible or not repeatable. Many variables can be measured and will behave predictably but there is some disagreement as to the underlying principles. The complexity is generally beyond mathematical modeling and organisms cannot be built at all. There is a great and growing body of knowledge but most of it is empirical rather than based on understanding of underlying principles. The third is psychology. There are several schools of thought with completely different models of what the mind is and how it works. There are few measurements that can be made and little agreement as to what it is that can be measured. Experiments are often not repeatable and subject to wildly different interpretations.
It is understood that skilled actors can portray any human behavior or mental state on stage and that this bears no relation to their "real" mental state. It is also understood that acting is not a talent restricted to professionals but possessed to some degree by most of us. Given these two facts we can't make certain statements about anyone's "real" mental state based on observed behavior. Perhaps there is no such thing as a "real" mental state separate from our behavior or a "real" self separate from the self we seem to be.
Suppose we ask how someone would be different if they did not have a self. What would you see? How would they behave? Self is partly made of memories but someone without memories is still presumed to have a self. Self is partly made of opinions but on any subject there are people with no opinion, they have selves. Would someone without a self be unconscious?
What of people with multiple selves? This can be a serious mental illness but perhaps most of us normally have many selves which we use in different contexts; the mental illness is actually a failure to manage the multiple selves or a failure of the selves to share memories.
Suppose someone who is near me a lot picks up on my behaviors and opinions; they come to imitate me in every way. They are acting like Harry Reid. Are they any less me than I am? After all I am only acting like Harry Reid. If I am my behaviors and someone imitates them then part of me has been reproduced in them. Just as we physically reproduce through sex and babies we reproduce our personalities, our selves, when people learn from us and become like us. Parts of the personalities of parents normally appear in their children.
If you know me well enough to simulate my response to many situations then a simulation of me lives in you. Is this simulation different from the one that lives in me? It is not exactly the same but both evolve and improve the same way. Perhaps you can become a better me than I am.
Suppose self is not a unified whole but is more like a cloud or bunch of clouds. You learn from me and part of me lives in you just as much as it lives in me. A ghost that visits us is just a part of someone's personality manifesting itself as an illusion.
I have repeatedly had an experience that I attribute to a small but separate self associated with my feet. I go barefoot almost all the time outdoors and there are sometimes fire ants where I live. On several occasions I have felt an ant sting on my foot and looked down to see the ant. Upon picking up the ant I discover that it is really just a bit of dirt that looks like an ant and could not possibly have stung me. The only way I make sense of this is that I saw the ant in my peripheral vision but did not notice it consciously. A separate self associated with my feet recognized the danger of a sting and simulated the illusion of a sting to get my conscious attention so I would remove the ant before it had a chance to sting. This suggests that part of my mind watches everything in my peripheral vision (and probably listens to background noises) and can create illusions to direct my awareness toward items of importance. This may be a very common process which normally operates so smoothly that only in exceptional cases can its mechanism be seen.
Counseling is supposed to be one of the disciplines that requires the most sensitivity and insight into human nature. Yet a program called "Eliza" was written very early in the history of computers which simulated a reflexive counselor by repeating back what the patient said in the form of a question. It noticed a few key words and would ask leading questions based on them. All in all it was a very short and simple program by modern standards. In spite of its absurd simplicity it was captivating and people would engage in long dialogues with Eliza which lead them to genuine insights and authentic self growth.
I predict that as we come to understand what personality, or self, or spirit, or soul, really is we will find that there is much less there than we had assumed. We will realize that our hubris and egotism have created the illusion of far more substance than we really have. Personality is nothing more than a loose accumulation of habits and behaviors. It reproduces when these habits and behaviors are copied by other people who then have parts of that personality as their own.