I just replaced the last parts on my 20-year-old "PC of Theseus."

To be honest, the case is still the original one, but almost every other part has since been replaced. Now, I’ve taken it back to the shop where I bought it 20 years ago and asked them to upgrade the motherboard, CPU, and memory - the last of the original parts.

So, is it still the same computer?

I also like that I can just keep replacing parts on an existing product rather than buying an entirely new device each time. That's exceedingly rare feature these days.

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If you kept the same case, I'd call it the same computer.
It's like a car: if you replace the seat covers, add a new air freshener, and replace the transmission, well, it's still the same car because the outside shell didn't change, just the bits inside it.
- Ok but if you break the front and back faces of your phone then send it in to get them replaced, is it still the same phone?
  I think what changes the identity of something is changing it's "brain", or how it operates. In the case of phones/computers, changing the CPU to a better one would change it's performance, so it would be a new thing. Same as with a car engine. If you have a 4-banger and change it to a V6 or even replacing it with a newer engine, it's now another car even if it's appearance is the same.
  
  To me, it's a percentage game. OP's computer is now 90% a different computer, your phone example is 20% a different phone, and your car is 30% a different car. The Ship of Theseus is 100% a different ship when it is fully swapped out, no matter how long it took.

Id consider mobo+cpu swap to be a new computer, and rarely if ever do just a cpu or just a mobo swap. Most other things id consider to just be moving the same build to a new case or an upgrade.
- Exactly, on a Theseus level, this pc still has the same deck, bow and stern, keel etc
  It's the same pc
  Amazing that everything still fits 20 years later

I used to do a lot of building, modding, overclocking, etc. I can't tell you why, but I always associated the motherboard as "the computer." If I replace the CPU, RAM, cards, cooling, drives, case, etc it's the same computer. And if a take a mobo out and put it in another chassis, that's now "the computer" or, at that point, "the old computer."
I had one 3/4 tower case that lasted me from 486sx, all the way to Pentium 3 and I still miss it, but I wouldn't say it was the same computer. The same case sat next to Moss's desk on The IT Crowd, and I'd get a little nostalgic seeing it.
- Because the motherboard defines it's capability.
- It's called the motherboard, so there's something in the name I think too.
- I feel the same. The motherboard determines what else you can fit in, like the chassis of a car. It determines the maximum GPU, CPU, ram, etc you can use.

Quantitative logic solves so neatly the Ship of Theseus problem.
Let's say that there are five essential components in a functional desktop: CPU, RAM, motherboard, internal storage (HD/SSD), case. Perhaps six if you count the screen (I tend to see it as something attached to the computer instead of part of the computer itself).
Before you took the computer back to the shop, the computer was 4/5=80% the same. (You probably swapped the storage, right?). After they upgraded the mobo, CPU and memory, it's now 1/5=20% the same, as only the original case remains.
...or alternatively pick some arbitrary component to say "when this one is replaced it's a new computer". That's what I do with the CPU, for naming purposes. (All my computers have names - Hollerith [retroactive name], Turing, Midgard, Tiberis [current])
I also like that I can just keep replacing parts on an existing product rather than buying an entirely new device each time. That’s exceedingly rare feature these days.
Ditto. And I wish cell phones were the same. Even if they were a bit bulkier as a result - it would mean buying less stuff pointlessly, it would be good for customers and environment.
- Yeah, the question would rather be 'when does it stop being the same thing?' It quite obviously no longer is if every single part has been replaced.
  Also, depends on what one means by 'the same computer.' The computer I've been using for the past several years mostly still remains. Some of the parts have been replaced a long time ago of which few have been there longer than the original.
  
  The question would be more like "how much of the same thing it is?".
  And kind of off-topic, but what are your current mobo/CPU/RAM specs? I'm asking because I did the same recently, changing quite a few parts of my computer.
- I think that logic actually doesn't hold up, perhaps because when a new piece is added, the PCs identity slowly changes. New pieces since it becomes part of the definition of what that computer is.
  Let's take the idea of adding a new piece, say a secondary drive. Does that make the computer a new computer? Of course not, that drive belongs to the whole. Does it make it 6/5? Technically not, since you're just counting the original pieces... even if said drive becomes integral to your PC by hosting you Linux distro you migrate to.
  Years pass, parts fail, and that Linux instance persists. Now you're down to 0/5 but somewhere along the way your PC of Thesius changed along with it's parts. Using that old definition makes no sense anymore. In fact, it never did. Some say it changed the night you learned about Arch Linux on Lemmy. Others say it changed when you left your Windows loving wife over her poor taste in OS.
  I... may have lost my train of thought. I guess all this is to say you can argue definitions all you want but there isn't a mathematical solution when we're taking about stuff like identity, definitions, etc.. but to be fair, it's a thought experiment, not meant to be solved so much as a way to provoke critical thinking.
  
  It's less that the identity of the PC slowly changes, and more that you give up assigning it a single identity. Instead you pick a point of reference (let's say, the PC as OP bought it), and then you measure how much it changed from then to now.
  That's how it works with quantitative logic - you never ask "is this the ship of Theseus?", you ask instead "how much of this entity is the ship of Theseus, as left initially in the Athenian harbour?"
  Let’s take the idea of adding a new piece, say a secondary drive. Does that make the computer a new computer? Of course not, that drive belongs to the whole. Does it make it 6/5? Technically not, since you’re just counting the original pieces…
  It can't be 6/5=120% - adding a secondary drive makes the computer slightly more different. It must be less than 100%.
  Since I'm counting long-term storage as 20%, and it changed halfway (the old drive is still there), I'd argue that now it's 90% of the PC that OP bought. (Of course, those numbers are simply made up, what matters is the reasoning.)
  even if said drive becomes integral to your PC by hosting you Linux distro you migrate to. [...] Others say it changed when you left your Windows loving wife over her poor taste in OS.
  This adds two interesting bits of complexity:
  Software is part of the PC. Data as a whole is. As such once you save a single file in a computer, the computer isn't exactly the same as it was before; similarity is now lower than 100%.
  Relevancy. A drive hosting your system is more important than one not doing it, even if both are being used. Changing the former should count more to decrease similarity than the later.
  but to be fair, it’s a thought experiment, not meant to be solved so much as a way to provoke critical thinking.
  Yup. It isn't something serious; just some millenniums old talk. As such losing your train of thought is not a big deal, it's part of the fun.

I have maintained such a PC and kept the old parts long enough that I rebuilt the original PC after fully upgrading and replacing it.
So I dunno what you call it.

Can't answer your question on a philosophical level but on an emotional/psychological level I would say that it's the same computer as long as it feels like the same computer to me, say by using the same desktop environment on the screen, and the same case, keyboard and mouse and if all parts have slowly been upgraded along the years not all at the same time as I would think that would create a kind of interruption in my relation to this PC.
To make a comparison, I bought my first SSD many, many years ago (to give you an idea, it was back when PATA still ruled). This SSD was tiny, it cost me an arm and a leg, and was not even that fast but even though I used it in the exact same computer I used with a classic HDD, the upgraded machine felt so incredibly much more responsive and so snappy that for me it was a new computer, and also a whole new experience.
Since that day, I've owned a few other machines, all SSD-based obviously, but never felt such a radical rupture while upgrading or even replacing a machine, no matter the CPU, ram or whatever else was new or better in it.

What do we define as "the same computer"? Is it that all of our data is there and it is set up exactly how we like it? Is it the components? Though we never directly interact with them after setup and some maintenance. What makes your PC recognizably your's?
I want to say no, its not the same computer. But what's the difference if you boot it up and all your stuff is exactly how you left it?
- I feel like the ship of Theseus is almost easier in that the components replaced are exactly the same. Replacing computer parts involve upgrades, unless you really are nuts and want to keep your 20 year old components lol I think once upgraded parts are installed I'd consider it "new" in that it performs completely different.

I don’t think it is personally but I think that depends on your thoughts about its character. To use an analogy, if you think of metal bands where swapping out members is quite common, I’d argue that the changes are much more noticeable than on something like a PC. The vocal or lyrical style changes, or the guitar solos, while still good lose something in their character.
Thinking of my own computers going from when I was 17 to the one I have now at 37, the technology is so vastly improved that the character would’ve been lost with most upgrades.
I think unless it’s a purely cosmetic change or maybe a storage upgrade, i.e. an improvement to existing components, it just isn’t the same.
That’s just me. This was a very interesting question.

I went full PC of Thesius within about 10 years, so kudos for sticking it out. And that includes the case and my vinyl stickers, but she still goes by GlaDOS and she's still my computer.
And yes, I never understand people too scared to upgrade their rig. I'll happily teach anyone willing to learn, since it's so easy lately.

I had my computer in an old AT case until like 2018 or so. I finally splurged and got myself a $60 Montech case with the power supply on the bottom. It's always been like that - swap out motherboard, cpu, ram, and toss everything else back in.

I vote for same computer. Mine is definitely still a Packard Bell. Each time I replace a part, that part became part of the original and I've never done a 100% replacement in a single session. I think the longest lived original part was the 3.5" floppy drive. I only remove it because I bought a case with no slot for an externally accessible drive.

If we swap a human brain are they still the same person? If you say no a brain makes the person then if you swap a computer's brain it is no longer the same computer. Even windows agrees because if you swap a cpu you must reauthorize your install.
- I'm happy to inform everyone, that I'm installing Linux instead

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