*warning* pub philosophy incoming
If the ancient Greeks had had bikes, Heraclitus, Plutarch and Plato would be discussing the Bike of Theseus, not the
Ship of Theseus.
Forget Einstein's riding-on-a-ray-of-light thought experiments, The Ship of Theseus thought experiment is the one that bakes my noodle
I *think* I come down on the side of the 'spatiotemporal theory', where gradual changes to a Thing don't prevent the thing still being the Thing... You can quickly get into ideas like 'the essence of that bike is still there' (but then Hobbes chucks a spanner in the works by asking 'what happens if someone gathers up all the discarded parts and makes a new Thing? Which thing is now The Thing?') They are, after all, just a collection of replaceable, inanimate parts, aren't they?
You can apply this to everything from brooms, classic cars and locomotives to 1960s bands and rock family trees, but I tend to apply it to vintage bikes. On the basis that lots of my bikes are older than me and will comfortably outlast me, I think any incremental changes I make to them (repainting, swapping out groupsets etc) are fair game, as their purpose is to be ridden. Similarly, if a like-for-like tube had to be replaced, then the bike is still the bike. However, they're '
only original once' so the 'mereological' tendency is to hold off from making any changes as long as possible!