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Theseus Paradox

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Paradox: The Ship Of Theseus

By James Donahue

This paradox stems from a Greek myth. It seems that Theseus was a great Greek explorer and sailor who returned home after sailing his ship from one end of the known world to the other.

After many epic adventures, a collection of Theseus’s possessions was gathered. Among them was his now historic ship. But the ship was old and in poor repair. Consequently his subjects set about to completely restore the vessel. They carefully tore away each board and replaced it with new timber. When they were finished the ship had been completely rebuilt, board by board. It looked exactly the same as the old ship. The lumber that had comprised the old ship lay in a heap on the ground.

And here we find the paradox. Was this still the Ship of Theseus, or was it a brand new vessel? Was the original ship lying in a heap of broken boards and timbers on the ground?

Suppose, just to make our point, that Theseus was unhappy with the fact that his old vessel had been replaced. So his loyal followers used the old wood pieces to rebuild the original ship, exactly as it was when Theseus arrived from his epic journeys. Now we have two identical ships, one old and one made of new material. Which one is the Ship of Theseus?

In this case it would be the old original ship. But if the old ship had not been rebuilt, could we say the new one is the Ship of Theseus? This is a philosophical dilemma that can be applied in yet another peculiar way.

Scientists say that our human bodies do exactly the same thing. All of the cells in our body are busy replacing themselves. Thus over the course of a decade or two, we exist in completely rebuilt bodies. So are we the same person once the body has been completely replaced?

In the case of living beings, the answer would have to be yes. Even though the body changes with age, and the cell replacement makes new bodies as we age, the spirit of the personality we are still exists within the body as long as we live. Thus we are, indeed, the same person.

In the case of the Ship of Theseus, the answer is that the completely rebuilt replica of the original ship is exactly that, a replica of the original. Only the original ship, made of the original parts, can be identified as the true Ship of Theseus.

It is true that wooden ships of old needed regular maintenance to remain afloat. Thus rotting wooden boards had to be constantly replaced as the vessel aged. But this did not change its identity. The old ships went into dry dock at regular intervals for what they called a "refitting." But they went back to sea as the original ship even though their identity might have been significantly altered.