The Hard Version
The Ship of Theseus
Replace every plank. Is it the same ship?
Now assemble the old planks.
WHICH ONE is the original?
The ancient Greek historian Plutarch posed a famous puzzle: the ship in which Theseus sailed was preserved by the Athenians. As planks decayed, they were replaced with new wood. Eventually, every single plank had been replaced.
The classic version asks: Is it still the same ship?
But the hard version - attributed to philosopher Thomas Hobbes - adds a twist that makes the paradox genuinely unsolvable:
What if someone collected all the old planks
and assembled them into a second ship?
Now you have two ships. Both have valid claims to being "the original."
They cannot both be the original.
This isn't a puzzle with a clever solution. It's a demonstration that "identity" isn't a deep metaphysical fact waiting to be discovered. It's a leaky abstraction we invented - and like all abstractions, it breaks down at the edges.
Watch Both Ships Form
Replace the planks one by one. Watch as Ship A transforms while Ship B is constructed from the discarded original wood.
Ship A: Continuously Maintained
The Original (?) Ship
100% original / 0% new
Spatiotemporal continuity: always the "same" ship
Ship B: Reconstructed from Old Planks
Waiting for old planks...
Material continuity: made of original materials
At every moment, Ship A was a ship - it was always seaworthy, always in use. Ship B was just a pile of planks until it was assembled. But Ship B is made of all the wood that was present when Theseus himself sailed.
Two Criteria, Opposite Answers
Philosophers have proposed various criteria for identity over time. The two most intuitive ones give opposite answers:
Spatiotemporal Continuity
An object is the same if it maintains continuous existence through space and time, even if parts are gradually replaced.
Verdict: Ship A is the original. It was never destroyed or disassembled.
Material Continuity
An object is the same if it is made of the same physical matter, regardless of what happened to that matter in between.
Verdict: Ship B is the original. It contains 100% of the original material.
Ship A (Continuously Maintained)
Ship B (Reconstructed)
Ship A is 100% original. Ship B doesn't exist yet.
The tracker above shows how these two criteria diverge as replacement proceeds. At 50% replacement, Ship A and Ship B have equal material claim. By 100%, their claims are exactly inverted.
Comparison of identity criteria:
| Criterion | Ship A | Ship B |
|---|---|---|
| Spatiotemporal continuity | Yes | No |
| Material continuity | No | Yes |
| Continuous function/use | Yes | No |
| Original form/structure | Yes | Yes |
| Causal connection to original | Yes | Yes |
| Legal ownership continuity | Likely | Disputed |
Neither ship clearly "wins" on all criteria. Different criteria point different ways. This is the core insight: identity is not a single objective property.
What Do You Think?
Before we discuss the implications, take a stance. There's no right answer - that's the point. Your intuition reveals which criterion of identity feels more natural to you.
Which ship is the original Ship of Theseus?
Notice how opinions divide. If identity were a deep fact about the world, we would expect more consensus. The division itself is evidence that we're not discovering truth - we're choosing conventions.
Identity Is a Leaky Abstraction
In programming, a "leaky abstraction" is a simplification that works most of the time but breaks down in edge cases. The concept of "identity" - the idea that things persist as the same thing over time - is exactly this kind of abstraction.
Why the abstraction exists:
- *For everyday objects, gradual replacement doesn't happen often
- *When it does, we don't usually reconstruct from old parts
- *The abstraction simplifies property rights, social relationships, planning
- *It matches our intuitions 99% of the time
The Ship of Theseus is a carefully constructed edge case that breaks the abstraction. It's not that philosophers are being pedantic - they're revealing that our everyday concept of "same thing" is not a coherent idea at the fundamental level.
"Identity is not a relation that holds between an object and itself. It is a convention we impose to simplify our model of the world."
This doesn't mean identity is useless. Leaky abstractions are still useful! HTTP is a leaky abstraction over TCP/IP. Object-oriented programming is a leaky abstraction over memory. We use these abstractions daily while knowing they can break.
Where This Actually Matters
The Ship of Theseus isn't just an abstract puzzle. It has direct implications for questions we actually care about - from brain cells to teleportation.
Your Body
Most cells in your body are replaced every 7-10 years
Rock Bands
Every original member leaves and is replaced over decades
Code Repositories
Every line of code is rewritten through refactoring
Companies
All founders leave, product changes, offices move
Teleportation
Star Trek teleporter: destroys you, recreates you elsewhere
Mind Uploading
Your mind is copied to a computer, biological body continues
The Teleportation Question
If Star Trek teleportation destroys you and creates an identical copy, is it transportation or murder? The Ship of Theseus shows why this question has no objective answer - it depends on which criterion of identity you privilege.
If spatiotemporal continuity matters: teleportation kills you.
If pattern/information continuity matters: you survive.
The same applies to mind uploading. If your consciousness is copied to a computer while your body continues living, which one is "you"? Both? Neither? The Ship of Theseus shows that our intuitive concept of personal identity wasn't designed for these scenarios.
What To Take Away
Identity Questions Are Not Factual Questions
When we ask "which is the real ship?", we are not asking about a fact in the world. We are asking which convention to apply.
Context Determines the Right Convention
For legal purposes, we might care about continuous ownership. For material analysis, we might care about original atoms. Both are valid in context.
Be Suspicious of Identity Essentialism
When someone claims an essential identity ("the TRUE version"), recognize they are choosing a criterion and presenting it as objective fact.
The Paradox Applies to You
Your cells replace. Your memories fade and reform. Your beliefs change. The "you" of 10 years ago shares neither material nor psychological continuity. Are you the same person?
The resolution to the paradox:
Both ships are the original Ship of Theseus under different criteria. Neither is the original in some absolute sense.
"The original" is not a property an object has. It is a label we apply based on contextual needs.
Identity is not discovered. It is decided.
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Reference: Plutarch, Hobbes (1655)