Are You the Same Person You Used to Be?

How much have you changed in the past 10 years? Your body has aged, and you have some different memories, beliefs, and attitudes. But many of your memories are the same, and your body has similarities and continuities with what it was before.

Will you be the same person 10 years from now? You might be dead, or have some kind of brain injury or dementia that will have taken away much of your mental functioning. Is a demented or unconscious self still you?

These questions raise the traditional philosophical problem of personal identity, concerning what makes people who they are. Philosophers usually address this question using thought experiments about imaginary happenings, such as brain transplants and teleportation. But such thought experiments are about as reliable a source of true conclusions as religious texts and Fox News. A more scientific approach to the self can better illuminate the problems of personal identity.

My multilevel mechanisms theory of the self understands a person as a complex system based on interactions at four levels — molecular, neural, mental, and social. A mechanism is a combination of connected parts whose interactions produce regular changes. For example, a bicycle has parts, such as the handlebars, frame, pedals, chain, and wheels, whose connections and interactions with your body enable you to ride it down the street.

Mechanisms change when they get new parts, such as a working wheel to replace a broken one, or when their connections and interactions alter to produce different changes: for example when a bicycle chain gets loose, making it hard to pedal. The identity of a mechanism is not all-or-nothing, but instead is a matter of degree, depending on how much the parts, connections, and interactions have altered. Similarly, there is no simple answer to the question of whether you are the same person you used to be because it depends on changes in four levels of mechanisms.

Four mechanisms of the self

Your molecular mechanisms have probably only changed a bit in the past 10 years. Barring mutations, you still have the same genetics based on DNA, but you have probably had some epigenetic changes in the chemical attachments that affect gene expression. You still have roughly the same neurotransmitters, but stress, depression, or good fortune may have affected the operation of ones such as serotonin and dopamine. Aging, maturing, or medication may also have affected levels of hormones, such as estrogen and testosterone.

Your neural mechanisms are probably still similar to 10 years ago if you haven’t had major problems, such as concussions or strokes. Your neurons still operate by exciting and inhibiting each other. You have lost some neurons from aging, but you have also gained thousands of new neurons every day. Overall, most of your 86 billion neurons are the same ones you had before, although new experiences and learning have modified the synaptic connections between them.

Mental mechanisms consist of representations, such as concepts and beliefs, that interact through inferences and other processes. Your mental changes include adding new concepts, such as binge-watch and transgender, and many new beliefs affected by changes in the world, for example concerning the state of the economy. You also may have changed your attitudes on issues such as politics.

The fourth level of mechanism relevant to the self is social because interactions with other people are central to human life. You may have gained some new friends or family members and lost others, or switched jobs or clubs. Such social changes can affect your mental representations, as well as your neural and molecular processes. For example, if you were in a very stressful romantic relationship 10 years ago, but now have a good lover, then your social improvement has affected your molecular mechanisms: less cortisol and more dopamine. Even if your neural mechanisms are compromised by dementia, you can still have important social relations through interactions with the people who continue to care about you.

Changes at all of these levels make it clear that you should not expect a simple, yes-or-no answer to whether you are the same person you used to be. You have changed in some ways, but not in others — mostly a matter of degree, but possibly a matter of kind, if you have had some catastrophic damage to your brain functioning.

Other views of the self

Other views of the self provide very different answers to questions about personal identity. If the self is a non-material substance impervious to physical changes, then you clearly are the same person you were. And you can even be that same person after death. Unfortunately, there is no good evidence for this.

Some philosophers from David Hume to Daniel Dennett have been skeptical about the existence of the self. If selves do not exist, then personal identity is highly problematic. There is nothing to hold together the mass of experiences and memories that people continuously acquire. My account is very different: There really is a complicated kind of personal identity by virtue of the ongoing molecular, neural, mental, and social mechanisms that constitute the self.

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