The question is this: who am I metaphysically?
The question of “Who am I?” is a fascinating one with many different types of answers which we may discuss at length in the future. The type of answer that I care about today is the metaphysical one.
Suppose I have an identical twin, and we have lived a good, long life, staying close to each other throughout our lives and we are now 80 years old. We are looking through old photos surprised at how far we’ve come. In many of the photos, it is difficult to tell us apart, and even now, sometimes our grandchildren mistake us for each other. In many ways, my old self is more similar to my old twin than my old self is to my young self, and my young self is more similar to my young twin than my young self is to my old self. Despite this, it is very obvious that my young self is me. My young twin and I share memories, interests, a childly form and more that I don’t share with my younger self, and yet it seems ridiculous to consider that my young twin is me even close to the extent that I, my older self, am me, my younger self. Why is this? What ties me to my younger self?
This example shows that genetics isn’t the tie. My identical twin and I share the same genetics, and he is not me. It isn’t physical similarity either. You may have heard of the Ship of Theseus thought experiment. Suppose there is a ship, that slowly has its pieces replaced until all of its pieces are eventually different from when it was first built. Is it still the same ship? You may have also heard of the idea that all of the atoms in our body are replaced over some time (about a year, even though some cells like brain cells last a lifetime). So we are like the Ship of Theseus. You could say that you, I and the Ship of Theseus are made of fungible parts.
Fungibility is generally used as an economic term, but I’m adopting it for this conversation. It means that something is totally interchangeable. Any ordinary $5 bill has the same value as any other ordinary $5 bill and two $5 bills have the same value as a $10 bill. This means that bills are fungible. In the same way, atoms and ship parts are fungible. Swapping them around doesn’t have an effect, so even if you are a lot different in a year, you won’t literally be a different person.
Alright, so we’ve established some intuition about this matter, but now it’s time to get into some grittier stuff. Namely, why is a figurative Ship of Theseus still itself after any amount of time at all? Does it perdure or endure? Perdurantism is the idea that physical objects have temporal parts. A perdurantist would suppose that <you> are a sort of four-dimensional “snake” that has one end at the moment you come into being, the other end at the moment you no longer are, and a thickness of the shape of you. Endurantism is the idea that you exist wholly in the present at every moment, but the identity of you “endures” from moment to moment.
Again I will leave you, Jonah, with this introduction and the statement that I agree with the idea of perdurantism as opposed to endurantism.
Awaiting your response,