We have as many definitions for “self” as there are disciplines which address it: biology, neuroscience, philosophy, law and legislation, ethics, spiritualism and religion etc.

Many of these definitions for “where the self ends and begins?” directly contradict each other; not only between disciplines but within the same one especially philosophy. Philosophical schools of thought have perhaps the largest discord between their ideas of what “self” is. It is a large part of philosophy to argue one or the other ultimately leading to a contentious mix of beliefs to a true definition for “self”. One that began thousands of years ago and is ongoing today.

But I would like to focus on “biological” self for a moment. One would imagine that because biology is not bound up in the metaphysical or subjective, it’s concept of self should be straightforward and non-contradictory.
And at face-value it seems so: the body is self. The environment is not.

But the immune system is built on a biochemical concept of “self”. Without it we would not be able to determine whether to have an immune reaction to “foreign bodies” or not. Our immune system knows that pathogens and materials from the outside world do not belong. And so mounts an immune response to dispose of them.

When a surgeon places a piece of metal or plastic or silicone in the body it rejects that substance “walking it off” or compartmentalising it away from the rest of the body. The same occurs with grafts and transplants unless we suppress the immune system.

This is fairly straightforward but what is not is “autoimmune diseases”.

Autoimmune diseases means the immune system has lost its concept of self vs. not self. It’s confused. It things a part of the body doesn’t belong even though it does and begins to attack itself.

A profound example is when the immune system attacks the brain - the supposed core and holder of “self”. If the brain is the seat of “self” the autoimmune attacks against it basically means your body does not believe your nervous system is part of itself. Multiple sclerosis or guillan barre barre syndrome are good examples.

This is why a believe “self” does not actually exist.
It is a useful term for language. But every discipline we know cannot pinpoint it as a solid universal and Working definition - both the physical and metaphysical. Self is an illusion materially and philosophically. Because if it wasn’t an illusion materially we would not have autoimmune disease.

This then raises the question of “what is the thing that is aware of its awareness?”
I’ve tried to fashion a definition;

“Self” is a utilitarian construct manufactured by biological organisms from the higher order organisation of intrinsic properties of information exchange, in order to discern between its constituents and thus conversely - its environment, and therefore modulate its behaviour.”

This definition supposes that self is not awareness but a product of awareness. And that awareness comes from some yet unknown property of information that can be cycled to form a stable system from which to “self” - reference (awareness/ act of conserving or changing the self)