Jon asked

You replace a defective part in your car. The next day you do the same thing with another part and then do so each day until you have replaced every part with a new one. Is what you have at the end of this process the same car with new parts or is it a new car? And if it is different when did it become so?

Answer by Craig Skinner

You raise a classical question as to how much compositional change (if any) an entity can undergo and still remain (numerically) the same entity. First raised by Plutarch as the repeatedly-repaired Ship of Theseus.

We could say that any change whatever destroys a thing’s identity (so called Mereological Constancy). Identity requires the selfsame atoms, none added, none taken away. So the new car exiting the showroom is already not the same car that sat there a moment ago — some of the tyre atoms have transferred to the floor, some new carbon atoms from the fuel are now part of the cylinder linings etc. As for people, you couldn’t shake hands with the same person twice. On this view, what we normally think of as an entity such as a car or a person, is really a vast series of fleeting different entities.

But we generally allow that things can change their parts without loss of identity eg I now wish to say I am the same man (now receiving a pension) as the young fellow who contributed to that pension from his wages years ago, and he saved because he thought the old boy who would benefit one day would be him, even though I am composed of completely different atoms from the me of 40 years ago. And similarly with artefacts such as cars or houses.

If we allow any part replacement at all, we are logically compelled to allow complete replacement of parts. This is because of the transitivity of identity (if A is identical with B, and B is identical with C, then A is identical with C) so that if its the same car after the oil filter is changed, it’s the same car after a tyre is later changed etc etc right up to complete replacement of parts. So, in short, the car at the end of the process you describe is the same car, as I am the same man at the end of my life.

However Thomas Hobbes (using the ship of Theseus example) pointed out a problem that arises if the old parts, instead of being binned, are kept, and then, when all the parts which constituted the original car are available, they are reconstructed into a car. Now we have 2 cars, let’s call them “Renovated” (Reno) which I have been driving all along, and “Reconstructed” (Reco) which has been built from the heap of spare parts accumulated over the years. Which car is to be identified with the original car?

There is something to be said for each option.
In favour of Reno: we routinely allow parts replacement; if the old parts had been discarded, we would have accepted Reno as a matter of course; and it’s the car I’ve been driving, taxing and fuelling all along.
In favour of Reco: if the original car had been disassembled/reassembled we would have accepted it as the same car as a matter of course; and, after all, Reco has all and only the parts of the original.

Possible solutions:

1. Both cars are the original. But this entails either one thing being in two places at once (not plausible) or that the original car was really two things which gradually separated (problematic – on seeing a single object, are we seeing one or two, or three or more, different things?)

2. Reno is the same car. Reco comes into existence only when assembled from old parts. Until then, these parts don’t belong to any car. If no renovation had occurred, these parts would have still belonged to the original car and we would have had one car throughout, initially assembled, later part-disassembled, later fully disassembled, finally reassembled. But a part cant belong to 2 cars at once. If my car’s oil filter is removed, it is still part of my (slightly disassembled) car, but once a replacement filter is fitted, the old one is no longer part of my car (or any other) although it might be reconditioned and become part of another car in the future – even one made of all and only the former parts of my car.

I prefer 2. to 1.

Answer by Shaun Williamson

It really doesn’t matter. Not every question has a definite or sensible answer.

Language was invented by human beings so that they could talk to each other about the things that interest them. It is as precise or as imprecise as we want it to be.