Man, if you only knew how easily my ego can be inflated…
I’m sure there’s a way of construing the identity of a thing such that we can say it remains constant. I think, however, that constant would have to be on a more abstract level. I think for sure our physical constitution changes, as does that of every other physical thing. The constancy of things can’t be in their structure or form or material content. I’m sure it can’t be mental either. If anything changes, it’s the mind. There’s not a single experience I can imagine that stays in my mind constantly without changing. Maybe constancy lies in continuity. If we know that a thing started out as A and gradually became B, we could say that A and B share a common identity. The butterfly is the caterpillar it once was. What would be constant in that case would be the thing’s history. It’s not like the butterfly can change so much it ceases to have come from the caterpillar, as if its whole past has changed.
Another thing that might preserve the identity of a thing might be a range of states (or properties, or structure, or whatever). So suppose I buy a new hammer. It’s all nice and shiny, full of bling, no rust. But over time it might accumulate rust. This might wear a bit at its integrity. But so long as its integrity remains within a reasonable range of functionality (i.e. it doesn’t break when I hit a nail with it) we can say it remains within the range require to be a functional hammer, my hammer. But if one day, it got so rusty that it just broke apart, it clearly would have falling outside the range of acceptable change, and you would say your hammer got destroyed, it no longer exists.
This idea of range works well with another theory of identity (not mine). Some philosophers say that we identify a thing based on its essential properties. This is in opposition to a things peripheral properties. For example, what makes a car a car? Is it the color? No, because I could paint my car a different color and I would still say it’s my car. Is it the noise it makes? No because if the muffler was damaged and it started making a loud noise, I would still say it’s my car. These would be peripheral properties. What would make the car essential to being the car? Maybe its form? Well, it could change form a little, maybe incur a dent from an accident, and I would still call it my car, but if it got crushed in a compactor or got blown apart in an explosion, would I still call it my car in that case? Maybe the form is an essential properties (i.e. without it, the car is no longer the car I identify it to be) but it requires change beyond a certain range of states in order for that property to be said to be gone.
There’s also function. So long as a thing performs the same function, we could say, it preserve the same identity. In my computer, there’s an adder (circuitry for performing addition). If I tell it: compute 2 + 2, it will tell me: 4. This circuitry obvious degrades with time. It gets worn from the heat, from corrosion, from physical bumps my computer incurs, but even after several years it still computes 4 when I give it 2 + 2. It’s not like the result slowly changes with the degradation, like after a year it starts computing 4.00001, after 5 years, 4.01, and so forth. So we could say that the circuitry remains an adder so long as the function doesn’t change, but again, function is an abstraction.
In any case, whatever it is that makes a thing constant, it has to be abstract. Physics is always in a state of flux. Even solid objects like rocks are constantly undergoing change when you look at them at the subatomic level. None of its particles remain still. But they do tend to reacquire their prior states. That’s how atoms are so stable. Though the electrons, protons, and neutrons that constitute the atom are always undergoing change and movement, they tend to push each other back to their prior states, or keep each other relatively close to the same state. On a macroscopic level, this gives the impression of a constant, an object that just sits there doing nothing. Maybe the fact of the particles always remaining close to this state is like the range of possible states a thing can move within while still being consider the same thing. As far as the mind goes, I would say there is way to much flux there to identify any constant, but there is recurrence. In order to identify a constant, the mind has to bring to consciousness the identity of the thing, but it’s not like that identity remains in consciousness permanently. Once the thing is out of sight, we stop thinking about its identity, but it comes back to mind later when the thing appears again. That’s recurrence, like the state of the atom recurring from the mutual influence of its particles. It might also be like function: though the neural circuitry of my brain obvious changes over time, the concepts it computes might be exactly the same. If we want to define constancy based on something more abstract–like continuity, essential properties, a range of values–then that too requires mental identification (abstraction is, by definition, mental) which is to say it can’t be there when we’re not thinking of it and so only makes sense as a recurring thing.
In the end, however, I don’t think all this matters that much. I’m quite satisfied to say that my favorite coffee mug is the same coffee mug I’ve had for years. Somehow, this works even if I know the particles that make up the mug are constantly changing and the mug itself slowly degrades over time. The reason why we need to think of certain things as constant seems to be met in most cases, including our identification of people, not least of which is ourselves. I definitely think it’s one of these things–continuity, function, range of states–that we go on when we identify things, including who we are, and so long as this works, there must be something we are anchoring our identifications onto even if that thing isn’t literally constant. In other words, I think there is a way to resolve this problem philosophically (which means you probably do get to say you have a constant ‘self’ within) but I would veer away from imagining that thing as literally an object that literally doesn’t change. It’s got to be something more abstract.