Imagine you have a ship that periodically requires you to replace old, worn planks with new ones to keep it in operation. After several years of use, do you still have the same ship, despite having replaced each of its parts one by one? Philosophers asked this question centuries ago, giving rise to the Ship of Theseus Paradox. It's an interesting problem, especially when we apply it to living beings. Humans, for example, replace almost all of our cells every ten years. Does this process turn us into new people?
The Greek Legacy
Many of the most interesting philosophical questions are thousands of years old. The Greek philosophers, for instance, were responsible for stating paradoxes that, 20 or 25 centuries later, still make us doubt our convictions. The Ship of Theseus Paradox is one of them. There is a Greek legend, collected by Plutarch, in which we can read:
The ship on which Theseus and the youth of Athens returned from Crete had thirty oars, and the Athenians preserved it from the time of Demetrius of Phalerum, since they removed the damaged planks and replaced them with new and more resistant ones, so that this ship had become an example among philosophers about the identity of things that grow; one group defended that the ship continued to be the same, while the other assured that it was not.
In other words: are we in the presence of the same ship if each of its parts has been replaced one by one? Perhaps the answer to the enigma lies behind the definition we adopt for "the same". Sometimes we consider that things can be qualitatively identical just because they have the same properties, and at other times they are numerically the same because they belong to a class.
For example, imagine you have two spheres that look identical. You can consider them qualitatively (though not numerically) the same. If you paint one of them a different color, it would be numerically the same as before, but it would no longer be qualitatively equal to its partner.
With that argument, the Ship of Theseus is qualitatively (but not numerically) different at the moment one begins with the repairs. The problem is that if we build our own definition of identity and make it broad enough, qualitative identity collapses into numerical identity. If one of the relevant qualities of the spheres in the example were their spatiotemporal location, then there would not exist two that - being in different places and times - could ever be qualitatively identical. This rationalization does not, of course, stop one from thinking about at what moment the Ship of Theseus ceases to be "the Ship of Theseus" to become its replacement. But there is an extra twist to this question.
Imagine that during the repairs made to the original ship, the workers in charge of the task save each of the old pieces they remove from the ship. After a more or less long time, everything will have been replaced and its owners (and we) will wonder whether it is the same ship or not. But what if the workers used the old planks to build the ship again? Would the ship thus built be the "Ship of Theseus"? There may be valid arguments to consider that one, the other, both, or none are the ship "original".
Locke's Sock
There are questions of this kind that are much more mundane than the ship problem. The famous English thinker, considered the father of empiricism and modern liberalism, John Locke, wondered whether a sock that has a hole, and to which we show our affection by mending it, is still the same after that operation. Almost everyone will answer yes, that it is effectively the same sock as always, albeit mended.
If another hole appears and we mend it again, logically the sock will still be the same, and if we continue with this task every time the sock shows its insides, sooner or later we will have one that retains none of its original material. Does the sock remain the same? At what moment does the sock cease to be the original?
In a more general form, the question seems to be if all parts of an object are replaced, does it remain the same? Setting aside the distances - temporal and other - that separate us from the Greek thinkers, we can use a passage from a book by Deepak Chopra to see the significance that an apparently trivial question like the Ship of Theseus Paradox can have:
With each breath we inhale billions of atoms that ultimately end up as cells of the heart, cells of the kidneys, cells of the brain, and so on. With each breath we exhale fragments and pieces of our tissues and organs, and we exchange them with the atmosphere of this planet. Studies of the radioactive isotope show that the body replaces 98 percent of all its atoms in less than a year. The body makes a new stomach lining every five days, new skin once a month, a new liver every six weeks, and a new skeleton every three months. Even our DNA, the genetic material that holds memories of billions of years of evolution, is not the same as it was six weeks ago. So if you think you are your physical body, which body are you talking about? The body you have today is not the same as the one you had three months ago.
The discussion is served. The person writing this text is not (or at least, his body is not) the same as the one who learned to ride a bicycle on a farm 45 years ago. And that one was not the same as the one who would spend afternoons in front of a Commodore 64 25 years later. However, all my friends still recognize me as the same (albeit fatter and with less hair). At some point in the future, perhaps within this very century, we will find a way to replace every part of our body, or even transfer our minds to a computer. Will we be truly immortal, or simply a group of murderers who "kill" a version of themselves to reincarnate into another?
In a way, the "gentle transition" that would involve replacing each of our cells with a new one, through a process that takes several months, does not seem to raise problems about whether we are still the same person. But if that process were carried out in a very small instant, many would think we had been changed.
Like all paradoxes, the Ship of Theseus makes us think. Strangely, a question raised centuries ago could have great importance in the future. If a millionaire of the 22nd century at age 100 undergoes a cell replacement therapy that gives him a new healthy and vigorous body, will his heirs have reason to claim that his relative has died and demand his assets right away? Or is he still the same, and entitled to enjoy life with his fortune and new body? Surely the laws will change over time to contemplate these questions, but interesting debates undoubtedly await us. And you, what do you think?
