1.1: Introduction to Philosophy and the Ship
of Theseus
1 Introduction to Philosophy and the Ship of Theseus
I have chosen to begin this book with a thought experiment to get the reader’s appetite
up for the projects of the Ancient Philosophers. What follows below is a topic that is still
discussed to this day, as its many potential resolutions each have some solid reasoning
to back up their acceptance. When I think that this philosophical problem of identity
was widely known and discussed in places such as Ancient Athens, I am full of
respectful awe at the fact that such intellectual interchanges supposedly occurred with
regularity as a part of social discourse for the betterment of all.
The Ship of Theseus1
The ship of Theseus, also known as Theseus' paradox, is a thought experiment that
raises the question of whether an object that has had all of its components replaced
remains fundamentally the same object. The paradox is most notably recorded by
Plutarch in Life of Theseus from the late first century. Plutarch asked whether a ship
that had been restored by replacing every single wooden part remained the same ship.
The paradox had been discussed by other ancient philosophers such as Heraclitus and
Plato prior to Plutarch's writings, and more recently by Thomas Hobbes and John Locke.
Several variants are known, including the grandfather's axe, which has had both head
and handle replaced.
Variations of the paradox
The classic paradox
This particular version of the paradox was first introduced in Greek legend as reported
by the historian, biographer, and essayist Plutarch,
The ship wherein Theseus and the youth of Athens returned from Crete had thirty oars,
and was preserved by the Athenians down even to the time of Demetrius Phalereus, for
they took away the old planks as they decayed, putting in new and stronger timber in
their places, in so much that this ship became a standing example among the
philosophers, for the logical question of things that grow; one side holding that the ship
remained the same, and the other contending that it was not the same.
— Plutarch, Theseus
Plutarch thus questions whether the ship would remain the same if it were entirely
replaced, piece by piece. Centuries later, the philosopher Thomas Hobbes introduced a
further puzzle, wondering what would happen if the original planks were gathered up
after they were replaced, and used to build a second ship. Hobbes asked which ship, if
either, would be the original Ship of Theseus.
Modern variations
John Locke proposed a scenario regarding a favorite sock that develops a hole. He
pondered whether the sock would still be the same after a patch was applied to the
hole, and if it would be the same sock after a second patch was applied, and a third,
etc., until all of the material of the original sock has been replaced with patches.
George Washington's axe (sometimes "my grandfather's axe") is the subject of an
apocryphal story of unknown origin in which the famous artifact is "still George
Washington's axe" despite having had both its head and handle replaced.
This has also been recited as "Abe Lincoln's axe"; Lincoln was well known for his ability
with an axe, and axes associated with his life are held in various museums.
The French equivalent is the story of Jeannot's knife, where the eponymous knife has
had its blade changed fifteen times and its handle fifteen times, but is still the same
knife. In some Spanish-speaking countries, Jeannot's knife is present as a proverb,
though referred to simply as "the family knife". The principle, however, remains the
same.
A Hungarian version of the story features "Lajos Kossuth's pocket knife", having its
blade and handle continuously replaced but still being referred to as the very knife of
the famous statesman. As a proverbial expression it is used for objects or solutions
being repeatedly renewed and gradually replaced to an extent that it has no original
parts.
One version is often discussed in introductory Jurisprudence and Evidence classes in
law school, discussing whether a weapon used in a murder, for example, would still be
considered the "murder weapon" if both its handle and head/blade were to be replaced
at separate, subsequent times.
In popular culture
The paradox appears in various forms in fictional contexts, particularly in fantasy or
science-fiction, for example where a character has body parts swapped for artificial
replacements until the person has been entirely replaced. There are many other
variations with reference to the same concept in popular culture for example axes and
brooms.
In Japan
In Japan, Shinto shrines are rebuilt every twenty years with entirely "new wood". The
continuity over the centuries is spiritual and comes from the source of the wood in the
case of the Ise Jingu's Naiku shrine, which is harvested from an adjoining forest that is
considered sacred. The shrine has currently been rebuilt 62 times.
Proposed resolutions
Heraclitus
The Greek philosopher Heraclitus attempted to solve the paradox by introducing the
idea of a river where water replenishes it. Arius Didymus quoted him as saying "upon
those who step into the same rivers, different and again different waters flow". Plutarch
disputed Heraclitus' claim about stepping twice into the same river, citing that it cannot
be done because "it scatters and again comes together, and approaches and recedes".
Aristotle's causes
According to the philosophical system of Aristotle and his followers, four causes or
reasons describe a thing; these causes can be analyzed to get to a solution to the
paradox. The formal cause or 'form' (perhaps best parsed as the cause of an object's
form or of its having that form) is the design of a thing, while the material cause is the
matter of which the thing is made. Another of Aristotle's causes is the 'end' or final
cause, which is the intended purpose of a thing. The ship of Theseus would have the
same ends, those being, mythically, transporting Theseus, and politically, convincing
the Athenians that Theseus was once a living person, though its material cause would
change with time. The efficient cause is how and by whom a thing is made, for
example, how artisans fabricate and assemble something; in the case of the ship of
Theseus, the workers who built the ship in the first place could have used the same
tools and techniques to replace the planks in the ship.
According to Aristotle, the "what-it-is" of a thing is its formal cause, so the ship of
Theseus is the 'same' ship, because the formal cause, or design, does not change, even
though the matter used to construct it may vary with time. In the same manner, for
Heraclitus's paradox, a river has the same formal cause, although the material cause
(the particular water in it) changes with time, and likewise for the person who steps in
the river.
This argument's validity and soundness as applied to the paradox depend on the
accuracy not only of Aristotle's expressed premise that an object's formal cause is not
only the primary or even sole determiner of its defining characteristic(s) or essence
("what-it-is") but also of the unstated, stronger premise that an object's formal cause is
the sole determiner of its identity or "which-it-is" (i.e., whether the previous and the
later ships or rivers are the "same" ship or river). This latter premise is subject to attack
by indirect proof using arguments such as "Suppose two ships are built using the same
design and exist at the same time until one sinks the other in battle. Clearly the two
ships are not the same ship even before, let alone after, one sinks the other, and yet
the two have the same formal cause; therefore, formal cause cannot by itself suffice to
determine an object's identity" or " [...] therefore, two objects' or object-instances'
having the same formal cause does not by itself suffice to make them the same object
or prove that they are the same object."
Definitions of "the same"
One common argument found in the philosophical literature is that in the case of
Heraclitus' river one is tripped up by two different definitions of "the same". In one
sense, things can be "qualitatively identical", by sharing some properties. In another
sense, they might be "numerically identical" by being "one". As an example, consider
two different marbles that look identical. They would be qualitatively, but not
numerically, identical. A marble can be numerically identical only to itself.
Note that some languages differentiate between these two forms of identity. In
German, for example, "gleich" ("equal") and "selbe" ("self-same") are the pertinent
terms, respectively. At least in formal speech, the former refers to qualitative identity
(e.g. die gleiche Murmel, "the same [qualitative] marble") and the latter to numerical
identity (e.g. die selbe Murmel, "the same [numerical] marble"). Colloquially, "gleich" is
also used in place of "selbe", however.
Four-dimensionalism
Ted Sider and others have proposed that considering objects to extend across time as
four-dimensional causal series of three-dimensional "time-slices" could solve the ship of
Theseus problem because, in taking such an approach, each time-slice and all four
dimensional objects remain numerically identical to themselves while allowing individual
time-slices to differ from each other. The aforementioned river, therefore, comprises
different three-dimensional time-slices of itself while remaining numerically identical to
itself across time; one can never step into the same river-time-slice twice, but one can
step into the same (four-dimensional) river twice.
Ship of Theseus (2012)
Plot
Summaries

The film explores questions of identity, justice, beauty, meaning and death through an
experimental photographer, an ailing monk and a young stockbroker.
—Recyclewala

If the parts of a ship are replaced, bit-by-bit, is it still the same ship? A celebrated
experimental photographer struggles with the loss of her intuitive genius as an unexpected
aftermath of a physical change; an intellectual monk confronting a complex ethical dilemma
with a long held ideology, has to choose between principle and death; and a young
stockbroker, following the trail of a stolen kidney, learns how intricate morality could be.
These disparate characters manifest philosophical dilemmas in their personal lives, but their
narratives converge to reveal an even larger fabric of connections, meaning, beauty,
existence and death in a delicately poetic finale.
—Recyclewala
The ship of Theseus, also known as Theseus’s paradox, is a paradox that raises the question of
whether an object which has had all its components replaced remains fundamentally the same
object or not.
Theseus is remembered in Greek mythology as the slayer of the Minotaur. For years, the Athenians
had been sending sacrifices to be given to the Minotaur, a half-man, half-bull beast who inhabited
the labyrinth of Knossos, Crete. One year, Theseus braved the labyrinth, and killed the Minotaur.
The ship in which he returned was long preserved. As parts of the ship needed repair, it was
rebuilt plank by plank.
The paradox was first raised in Greek legend as reported by Plutarch
“The ship wherein Theseus and the youth of Athens returned from Crete had thirty oars, and was
preserved by the Athenians down even to the time of Demetrius Phalereus, for they took away the
old planks as they decayed, putting in new and stronger timber in their place, in so much that this
ship became a standing example among the philosophers, for the logical question of things that
grow; one side holding that the ship remained the same, and the other contending that it was not
the same.”
The question raised by Plutarch – “Does the ship remains the same even if every piece is replaced
by a new one?” is quite interesting and puzzling at the same time. Centuries later, the philosopher
Thomas Hobbes added to the puzzle, wondering: “What would happen if the original planks were
gathered up after they were replaced, and used to build a second ship?”
If this happened, then, then it would seem that Theseus had returned from Knossos in two ships.
First, there would have been Theseus’ ship that has had each of its parts replaced one by one.
Second, there would have been Theseus’ ship that had been dismantled, restored, and then
reassembled.
Each
of
them
would
have
been
Theseus’
ship.
Theseus, though, sailed in only one ship. Which one?
The Greek philosopher Heraclitus attempted to solve the paradox by introducing the idea of a
river where water replenishes it. Arius Didymus quoted him as saying “upon those who step into
the same rivers, different and again different waters flow”. Plutarch disputed Heraclitus’ claim
about stepping twice into the same river, citing that it cannot be done because “it scatters and
again comes together, and approaches and recedes”.
One common argument found in the philosophical literature is that in the case of Heraclitus’ river
one is tripped up by two different definitions of “the same”. In one sense things can be
“qualitatively identical”, by sharing some properties. In another sense they might be “numerically
identical” by being “one”. As an example, consider two different marbles that look identical.
They would be qualitatively, but not numerically, identical. A marble can be numerically identical
only to itself.
One common argument found in the philosophical literature is that in the case of Heraclitus’ river
one is tripped up by two different definitions of “the same”. In one sense things can be
“qualitatively identical”, by sharing some properties. In another sense they might be “numerically
identical” by being “one”. As an example, consider two different marbles that look identical.
They would be qualitatively, but not numerically, identical. A marble can be numerically identical
only to itself.
Infact some languages like German, for example, “gleich” (“equal”) and “selbst” (“self-same”)
are the pertinent terms, respectively. At least in formal speech, the former refers to qualitative
identity (e.g. die gleiche Murmel, “the same[qualitative] marble”) and the latter to numerical
identity (e.g. die selbe Murmel, “the same[numerical] marble”). Colloquially, the terms are
sometimes used interchangeably however.
The Ship of Theseus and Personal
Identity
What makes you you? Such a simple question is surprisingly hard to answer. (Tweet
this.)
The Ship of Theseus is a classical philosophical puzzle about personal identity. I’ll
give the quick version. (Note: if you’re a professional philosopher, you will not be
happy with the quick version. But you likely won’t be happy with much of anything I
write here, anyway.)
Imagine you have a wooden ship in your backyard and decide that one day you want
to take it apart piece by piece. You go about taking it apart, delicately removing each
part as if you were going to use those same parts to rebuild it later (should you decide
to do so) and storing it in your garage.
Now, here’s part of the problem. When you remove each piece, it still seems to be
(intuitively) the same Ship. Removing one plank from the floor, for example, doesn’t
seem to make it a different ship. However, if you keep up with the process, you will
end up with all of the pieces of the Ship in your garage, but it will no longer be the
Ship of Theseus, since, by hypothesization, you won’t have a ship – you’ll have a pile
of wood that used to be a ship.
But at some point in your deconstruction, the Ship had to move from existence to nonexistence, unless you want to say all the pieces in the garage is the Ship. At what
discrete point did the Ship cease being the Ship?
A further wrinkle: suppose that, rather than just tearing the Ship apart, you decide to
replace every wooden piece you removed with an aluminum piece of the exact same
dimensions. So, when you start, you have a completely wooden Ship, but at the end,
you have a completely aluminum Ship. But, at each discrete stage of time, you only
have a ship that is one piece different than it was in the previous moment.
An even further problem: suppose that you decide to use the wooden planks you
removed in the case above to build another Ship which is materially identical to the
original ship. At the end of that project, you’ll have two Ships, one aluminum and one
wooden, that each have a claim to being the Ship of Theseus. They can’t both be THE
Ship of Theseus, but it could be true that they both could NOT be the Ship of Theseus,
but the problem becomes, when was the Ship of Theseus destroyed?
A few options:
The Ship of Theseus is what it is by the individual parts that make it up.
In this case, the second you removed the first wooden plank from the Ship, it ceased
to be. But the individual atoms in the wood are forever changing, with the result that
the Ship is never itself.
The Ship of Theseus is what it is because of its structure.
In this case, the Ship remains the same ship throughout the change from wood to
aluminum, so you have the seemingly contradictory result that the Ship of Theseus is
both an aluminum and a wooden ship. Furthermore, when you have two ships (as in
the last thought experiment), they both have identical structures, so you wind up with
the result that both are THE Ship of Theseus – meaning that two discrete things are
one numerical thing.
The Ship of Theseus is what it is because of its history.
In this case, the Ship remains the same because of its particular role in the history of
the world. Parts come and go, but the actor remains the same. You’ll still wind up
with the problem in the case of Theseus duplicates, because each share a relevant
history with the “original” ship.
The Ship Represents Us
Now, the real problem is not at all about ships, but instead about that that makes us
who we are. We know that parts of who we are changes from year to year, but we still
think we’re the same people.
Is it because of our parts — i.e. the individual matter that makes us up? Breathe,
and you’re no longer the same you.
Is it because of our structure? Lose a limb or cut your hair and you’re no longer the
same person.
Is it because of our history in the world? Were you to be duplicated, you’d either
have a existential twin, or you’d cease to be.
Is it because of our thoughts, feelings, and all the other stuff that goes on in our
heads? Lose your memories, and you’re no longer you. Have a radical change of
heart, and the person you were once before is gone.
Is it because of our souls? Souls could logically be duplicated and would run the
same risks of the Ships. (Sidebar: soul talk, in general, is a way to understand what it
is that is us through time. Our physical bodies come to pass and theories of personal
identity that posit spiritual existence before and after physical existence have to have
some device to allow for identity through the different phases of our existence.)
This ancient thought exercise will
have you questioning your identity
that you get a new body every 7-10 years. The reason is, different cells last different
lengths of time. Colon cells last only about four days, skin cells two to three weeks,
and neurons a lifetime. In that, the body you are in now is not the same as it was last
week, last month, or last year, although it feels the same. So are you truly the same
person?
It may predate him, but the ancient Greek historian Plutarch was the first to write
this intriguing thought exercise down. It’s called the Theseus Paradox or the ship of
Theseus. A hero of Greek mythology and the supposed founder of Athens, Theseus is
said to have won a number of naval battles and defeated several monsters,
including the Minotaur. Plutarch’s recording of the memorial set up to honor this epic
founder has had great thinkers scratching their heads for millennia since.
The ship wherein Theseus and the youth of Athens returned had thirty oars, and
was preserved by the Athenians down even to the time of Demetrius Phalereus, for
they took away the old planks as they decayed, putting in new and stronger timber
in their place, insomuch that this ship became a standing example among the
philosophers, for the logical question of things that grow; one side holding that the
ship remained the same, and the other contending that it was not the same.
Model of a Greek trireme. Credit: Deutsches Museum, Munich, Germany.
The memorial remained floating in Athens harbor for centuries, according to Plutarch.
It’s said to have been erected during the lifetime of Demetrius Phalereus, a famous
statesman and writer, who was thought to have walked the earth around 350-280
BCE. Over time, the ship’s boards began to rot. Like with the upkeep of any important
monument, these were replaced.
At some point, all the boards in the ship must’ve been switched out. So the vital
question is, is it still the same ship? If not, at what point does it cease to be the
ship of Theseus? Replacing one board may not be such a big a deal. But what
about when half of them are no longer original, most of them, or all of them? In a
way, human life is like this.
Some philosophers say we are our body. But as we grow older, we get wrinkles
and gray hair. We develop health problems and modify our diet. We change our
look, our clothes, our hairstyle, and more. And then there’s the death and
replacement of most of our cells. So our body isn’t a permanent fixture. Others
say our mind is what gives us our identity. Yet, our outlook on life changes over
time. We may become an addict or get sober, find religion or lose it, just as we
maybe become jaded... or embrace an unbridled optimism.
There are those who say our title, occupation, position, and responsibilities lend
us identity. And even so, we may have different friends in years to come or a
different spouse. Our children will grow up and move away. And our career
likely will not stay the same, either. On average, a person changes careers 5-7
times in their lifetime and has 11.7 jobs. That’s impressive, considering how
much of our identity is derived from our work and career. We may move to
another city or even the other side of the world. So with all of these changes, are
we really the same person throughout? If not, which version is the real you? Is
there one? There are many ways to answer.
Unlike a statue which is frozen in time, one of the only real constants in human life is
change. But if that’s so, then what is our true identity and how may we get to know it?
Credit: Getty Images.
Some philosophers say changing one plank alters the ship irrevocably. Others say if at
least one original plank is still in the ship, it’s the real deal. Then there are those who
posit that the monument remains Theseus’s ship throughout. Try as they might, it’s
hard to pin down exactly why a gradual change is okay, while a sudden one feels
unacceptable. Consider how shocking it is when someone has an accident, say a head
trauma, and they wake up a completely different person. Yet, if they made the same
changes gradually over time, we wouldn’t find it particularly jarring. One common
answer is known as Spatio-temporal continuity.
Volume 90%
This says that all objects change continuously as they travel through space-time. As
such, they’ll have parts replaced from time to time, may even change form or
composition. Yet, their identity remains. The famous Greek philosopher Heraclitus
posited that we don’t step in the same river twice. Everything is constantly changing,
though we’re not likely to notice. The equivalent in our species can be summed up
very well in a statement by Harvard psychologist Daniel Gilbert. He said, “Human
beings are works in progress that mistakenly think they’re finished.”
Many philosophers have put their own spin on the Theseus paradox over the years.
17th century English philosopher, scientist, and historian Thomas Hobbes, for
instance, wrote a version where a collector took all the original boards that were
replaced and one-by-one, built a second ship of Theseus. So which ship is the actual
one, the immaculate memorial in the harbor or the rotting one in the dry dock? There
is no right answer. What this paradox reminds us is, although we see identity as a
fixed and solid structure, it’s actually thin, malleable, and ever-changing.