The Ship of Theseus

In Greek legend, the hero Theseus sailed to Crete, killed the monstrous Minotaur and returned home to Athens in triumph. Centuries later, the Greek writer Plutarch recorded what became of the hero's ship. The Athenians preserved it for generations, and whenever one of its planks began to rot, they removed it and fitted a sound new one in its place. As a result, Plutarch noted, the ship became a famous example in philosophical debates about things that grow and change: some thinkers insisted it remained the same ship, while others argued it did not.

At first glance, the puzzle seems easy to dismiss. Replace a single plank, and surely nobody would claim that a different ship has suddenly appeared. But if one new plank makes no difference, it is hard to see why a second should, or a tenth, or a hundredth. Sooner or later, every piece of the original timber has been replaced, and we are left looking at a ship that shares not a splinter with the one Theseus actually sailed. Something seems to have been lost along the way, yet there is no obvious moment at which the loss occurred.

In the seventeenth century, the English philosopher Thomas Hobbes made the problem considerably more awkward. Imagine, he suggested, that someone had collected all the discarded planks and later reassembled them, in their original order, into a second ship. We would then have two rivals for the title. One has stood in the harbor all along, repaired piece by piece but never taken apart. The other is built from the very wood that once carried Theseus across the sea. If they cannot both be the ship of Theseus, which one has the better claim?

The puzzle might sound like an idle game for people with too much time on their hands, but versions of it surface wherever we look. Consider a band that has toured for forty years, although none of its founders still plays in it. Fans may argue bitterly over whether it is really the same band, and disputes over the name can even end up in court. Companies, too, can outlive every employee who built them, and some cities have been destroyed and rebuilt so thoroughly that hardly a stone of the original remains. Yet we go on calling them by the same names.

The most personal version of the puzzle, however, concerns our own bodies. A popular claim holds that every cell in the human body is replaced every seven years, but this turns out to be a myth. Cells are renewed at very different rates: those lining the gut or forming the outer layer of the skin are replaced within days or weeks, whereas most of the neurons in the brain last a lifetime. Still, much of the material you are made of today was not part of you a decade ago, and yet you would hardly conclude that someone else is reading this sentence.

Philosophers have offered several ways out of the puzzle. One answer locates identity in continuity: an object remains itself as long as it changes gradually, keeps its overall form and goes on playing the same role, which would make the repaired ship the genuine one. A second answer gives priority to matter, and therefore to the reassembled ship, on the grounds that its planks are the ones Theseus actually stood on. A third approach suggests that there is no hidden answer waiting to be found. Once we know exactly what happened to every plank, there is nothing further to discover. We simply have to decide what we mean by "the same" in each case, and different purposes may justify different decisions.

This last view can sound like a cop-out, but it captures something important. A museum curator, a lawyer and a sailor might give different answers and all be right, because each cares about a different kind of sameness. The ship of Theseus does not show that identity is an illusion; rather, it shows that the word "same" carries more weight than we usually notice. When we ask whether something is still the same, we are often really asking what we value about it: its material, its shape, its history or its purpose. Perhaps that is why a puzzle about a rotting ship has stayed afloat for so many centuries.