In Greek mythology, the hero Theseus, who slew the Minotaur and escaped its maze, returned from Crete to Athens where the Athenians preserved his ship in seaworthy state for more than a thousand years. It was an emblem of courage and a reminder of a national hero that many Greeks considered more legendary than mythological.
The Ship of Theseus was carefully maintained. Parts that rotted away were replaced with exact replicas. And in a ship made almost entirely of wood, crude iron, rope, and sail, everything rots, so eventually everything gets replaced.
Which makes the identity of the ship an interesting question.
If none of the original parts remains — let’s say even the metal bits have corroded and been replaced — it’s hard to claim it’s the same ship that sailed from Crete to Athens.
To make this obvious, imagine that some point after its return, the entire ship is carted away, and an identical replica moved into its place. The maintenance just accomplishes the same thing piecemeal over a period of time.
To make this interesting, imagine that one of the maintenance workers kept all the replaced parts and rebuilt the original ship in his backyard.
(Alternately, consider the original ship carted away to that backyard.)
When seen as two distinct ships that never touch, it seems clear that the original ship remains the actual historical artifact while the other represents, or stands in for, the original.
It’s less clear when the ship is replaced piece by piece, because when does the original become the copy?
At what point does the one in the backyard become the historical Ship of Theseus?
Does the order of part replacement matter? Is there a key part (the keel or the mainsail) that carries the ship’s identity? Does it matter if the parts are discarded or never assembled?
There is a modern variation on this; it’s often called My Grandfather’s Axe.
That version usually goes something like this:
My grandfather had a wonderful axe that he handed down to my father. At some point, my father had to replace the handle, which he did with an exact replica (same type of wood and everything). I inherited the axe, and recently I had to replace the axe blade (which I did with an exact replica).
Now nothing of the original axe remains. Is it really my grandfather’s axe? What I now hold in my hand has been “the family axe” since my grandfather bought it (and killed his first vampire with it).
If it’s a copy, did it become a copy when my father replaced the handle or when I replaced the blade? Is it the continuous role of Grandfather’s Axe that matters?
Not to be outdone, my favorite science fiction author, Terry Pratchett, tells the axe version in a Discworld story (The Fifth Elephant) that considers a sacred political object that is mysteriously stolen (from a locked room!) but later recovered.
Or is it? It appears the recovered object is a copy.
And it turns out that the one that was stolen was also a copy.
Which makes perfect sense when you think about it.
Even very large scones, even of dwarf bread, do not last for centuries. Of course it’s a copy. Had to be.
But the role of the Scone of Stone, that never changes, the role endures!
Which brings us to the idea of continuity.
The Dwarf’s Scone of Stone, the Greek’s Ship of Theseus, the family axe, along with state roles like Presidents and Kings and Queens; these are continuous roles that are generally not fulfilled by the same continuous physical entity.
The old phrase, “The King is dead! Long live the King!” (which confused the crap outta me when I was young) speaks to how kingship transfers from person to person.
(Which isn’t really a challenge on Discworld where light moves leisurely — even sluggishly in the morning.)
Here’s what makes all this interesting…
Physically, humans are replaced entirely every seven years or so. We have a new pancreas roughly every 24 hours. (This doesn’t happen all at once, obviously, but cell by cell — as if maintenance workers were replacing the rotted bits.)
This includes our brain cells, of course. Even the machinery of the mind is replaced.
Yet our minds seem continuous. Our patterns of consciousness do evolve and change over time, but we seem to have a continuous awareness. We have what is sometimes called a unitary experience.
Recently the YouTube video below got me thinking about the supposed paradox of Theseus’s Ship, and while I don’t see much actual paradox in the physical example (it depends on original material versus defined role), it did generate some speculation about our minds.
(Which is really the whole point of the Ship of Theseus metaphor.)
For a moment, it occurred to me that the spark of continuous awareness might be evidence of something like a soul or, at least, an indication that the human mind is unique in interesting ways.
(Put that way, almost anything can be seen as interesting. If you can find something that’s truly not interesting, that is interesting.)
But imagine the following:
There is a computer designed to calculate the digits of pi. Which go on forever, so this computer cranks out pi digits until it finally dies of old age.
In the meantime, it is maintained in good condition, and many parts can be (and are) replaced to keep it running. Even the algorithm that calculates the next digits can be patched to make it more efficient if a new method is found.
At some point vital parts can no longer be maintained, the machine dies, and the calculation stops.
If, as many believe, our minds are the result of a calculation, then this is a good analogue for what happens with humans. But (it is supposed) our minds don’t calculate pi; they calculate consciousness.
In the pi computer, even if it’s strictly a hard-wired computer, and even if it’s a mechanical calculator, we’re very clear on what comprises the algorithm and what comprises the matrix for the algorithm.
That’s the missing piece with software AI.
We have no idea how to separate the mind’s supposed “software” from the brain’s obvious hardware.
(Crucially: It may not be possible. Mind may supervene on the entire physical system. I am decidedly among those who believe mind is not an algorithm. See Information Processing.)
Either way, it’s interesting to think about the continuous «I» that each of us lives with. Just what is that core of personality who survives so intact all the years of our life as our body changes and ages?
Are we not, in so many ways, the same person we were as a kid, or in high school, or in college? And having learned and experienced, are we not, in so many ways, that same person?
How we change, what new things we take on board; and how we stay the same, what things we try to retain; these are a big part of what defines our character, nature, and personality.
So build and maintain your ships wisely! Use your grandfather’s axe.