Aristotle said that explanations should include a material cause. By this he meant whatever substances are needed for a particular event or thing to happen. Suppose I ask the question, ‘What makes you a unitary personality and allows you to persist as a distinct individual?’ Those who are a little science-minded, or a little fond of the obvious, might say: “Well I’m enclosed in this body. It has a boundary between me and the world around it.” Good for you. That is indeed your material cause.
These days we know that our material uniqueness starts with our DNA, which is also becoming the ultimate ID badge. DNA is a necessary cause of individuality. It is not however a sufficient cause, because each of us also have a unique environment that also affects how we develop. Note that regardless of how many writers try to dumb down the concept, DNA is not a “blueprint”. The closest analogy might be a computer program optimized over and over so that each piece of it is used in numerous and interacting ways. Discarded sections are left in but tagged as obsolete. Random parts are repeated and randomly mutated. Then the computer code is run through a compression algorithm to pack it down, and then it’s encrypted. Placed in a sack of nutrients, it somehow unpacks itself, creates its own interpreter, and runs its program, with constant feedback from the environment (the cell, the body, the encompassing society) affecting the execution and indeed modifying, through epigenetics, the program code itself. There’s different outcomes for different tissues, as well, while the random changes to the working interpreters inside our zillions of cells continue throughout life. That’s how not simple DNA is.
So how do we extract our unique identity from the DNA program? The theoretical biologist and Buddhist mystic, Francesco Varela, has perhaps the most poetic account. His concept is called autopoiesis: how living systems self-organize so that the mechanisms of the system are encapsulated in a boundary that is created by those same mechanisms, and the boundary is what enables the system to function. You see the circularity, almost a self-referencing definition there? Well …it might go down easier with some concrete examples. Varela says that each person has three distinct, but coexistent autopoietic systems: cellular structure, the nervous system, and the immune system. Cells of course have a membrane defining their extent, but they also get together and form the largest organ in the body, the skin. The skin is the bag that holds us together physically and acts as both barrier and transceiver to the outside world. So it clearly is one material cause of our physical integrity and persistence over time.
The second autopoietic system is the immune system. It’s very nature is to define the difference between self and non-self, and to enforce that as necessary. While it uses the body as its boundary, it also is a virtual boundary, repelling or disposing of invaders. It’s like Gandalf telling the Balrog, “You shall not pass!” (If only there was a non-copyrighted image to stick here!)
The third autopoietic system, the nervous system, has the brain as its extremely high-value, high-function core. Your brain is bounded by the cup of the skull as its primary physical protection. It also has a triple outer layer of membranes, as well as a complex of tightly fused cells around most of its blood vessels, the so-called blood-brain barrier. The penetration of these boundaries is necessary for direct computer to brain interfaces. This is why, even though such interfaces have great promise, and indeed are helping people already, most folks of any sense would find them creepy and even frightening.
And the brain does so much more for our integrity and continued existence. It guides our behavior (usually, hopefully) attempting to ensure that we survive and thrive. It also actually constructs the first-person Self — your very “I” that is your phenomenological essence — as part of a brain-made internal model of the perceived and understood world. This model sets us off (yet another boundary) from whatever ineffable cosmic reality it is that actually surrounds us.
So we have these boundaries, and uniquely programmed biological machinery, that preserve some aspects of an enormously complicated physical pattern over our lifetimes. Is this sufficient to take care of our self continuity? Not really. Any vertebrate animal has of the above boundaries. Some might even have the rudiments of a Self. When we think of ourselves as a person having an identity we are talking about much more, including a self concept (the answer to Who am I?) and the above-mentioned first-person conscious point of view. Philosophers have really dug their teeth into persistence of the Self.