The Ship of Theseus

From The If Machine by Peter Worley

When doing The Ship of Theseus chapter from The If Machine make the puzzle visual by assembling a model of the ship on the floor with some identical pencils and a piece of paper for the sail. As you explain the process of gradual change replace each pencil with another pencil creating a pile of the discarded pencils to one side of the ship. Invite the group to come up and use the model to demonstrate their ideas for the benefit of everyone in the group.

This extension to the session already in the book helps to make the exercise visual and kinesthetic so that other learning styles in addition to the auditory can be accommodated.


This example was famously supplied as a philosophical thought experiment by Thomas Hobbes, the British philosopher of the English civil war, into which he was born (he said that he was born twins with fear), but he drew the example from Plutarch, a Roman writer, and, of course, Theseus comes from Greek mythology.

In order for this session to be philosophically fruitful it is necessary to understand the philosophical subtleties involved in an exploration of the thought experiment. The embedded question to bear in mind in this whole discussion is if it is a new ship when all the parts are replaced then at what point does it become a new ship? This is where a lot of the philosophy will lie because here we are faced with the ‘problem of vagueness’.

If it is a new ship when the parts are all replaced and only then, would that mean that when it only had one part left to replace, it was still the old ship? If so, this seems a little odd. If not, then when does it become the new ship? This particular problem is known as the sorities paradox (from the Greek word for ‘heap’): how many grains of sand make a heap?

For the purposes of keeping things clear for the discussion it is a good idea to illustrate all this with examples as you go. I would ask the children to imagine that the ship has 100 parts. You can then use this at each point of the discussion to have them explore and reach the difficulties for themselves. For example, if someone says that it would be the new ship when more than half the parts are replaced, test it: so are you saying that it is a new ship when it has 51 metal parts and only 49 wooden ones? Then ask everyone what they think about this.

The Story

Theseus owned a ship and the ship was entirely made of wood. Every time a piece of the ship needed replacing it was replaced with a metal part. This went on for a few years until eventually it was entirely made of metal.

Task Question

Is the metal ship of Theseus the same ship as the wooden ship of Theseus?

It may be helpful to draw a diagram similar to this as you explain the scenario:

There is very likely to be materialists aboard your classroom, that is to say, those that will maintain that the ship is different if it is made of different stuff (“it was made of wood but now it’s made of metal!”) Thomas Hobbes was also a materialist in his response to the problem. He proposed a version of the following thought experiment to anyone who would entertain that the metal ship is the same as the wooden ship:

On board the ship was a sailor who really wanted his own ship but could not afford one. So, he came up with a plan: every time Theseus decided to replace one of the wooden parts of his ship with a metal part the sailor would take the discarded piece of wood and hide it in his shed. When eventually he had collected all the wooden parts he re-assembled them into a ship again.

Task Question

Does this mean there are two ships of Theseus or one? (Again, use diagrams to explain all this)

The sorts of ideas you might expect to hear in this session are as follows (often in different words):

* That the ship is different the moment the first plank is replaced because any change would result in a different ship. Later, when you move to a discussion of personhood, a response question to this point could be: does that mean that any change to myself/yourself, such as a tooth falling out, makes me/you a different person?

* That the ship is different only when the last piece has been replaced because only then is there none of the original ship left. Response question: does that mean that when there is only one piece of the original ship left it is still the old ship?

* If it suddenly changed into metal then it would be different, but if it changes gradually then it is the same ship because, at each stage of change, it is related to the old ship in that it is only minutely different.

* Even though the materials it’s made of change, the shape, the name and the design stay the same, so it is the same ship.

* It is the same ship if other people think it’s the same ship.

The Self of Theseus

At some point you will want to talk about how the discussion of the ship pertains to how we think of ourselves. This will either happen very naturally when the children start to make the connection, or, you will need to make the connection explicit yourself. Here are some suggestions of how it can be done:

Show two photographs next to each other of a person as a young child and as an old person. Ask the children whether they think they are the same person and why.

If you are with older children (age 10 upwards) then you can explain how scientists tell us that our cells are completely replaced every 7 years or so and then ask the children if this means that they are a different person every 7 years.

Nested Questions 

What is it that makes us the same person through time?

Possible responses to this problem are as follows:

* People and things are different. Response question: how are they different?
* People have thoughts and memories but ships don’t.
* We might change on the outside but our personalities stay the same. Response question: does that mean that our personalities can’t change?

Each of these insights can lead to further related discussions in themselves.

For the philosopher John Locke (1632-1704) it is that we are linked by memory to our past selves that makes us the same person through time. So, for Locke, it is not our body that makes us the same person – as this is constantly changing – but our mental life that makes us a person that lasts through change. Response question: if we lose our memory would that make us a different person?

Online philosophy

Sorities paradox

Concrete and personalised discussions

When doing philosophy with children they may begin to lose interest if the discussions remain too abstract or irrelevant to their own experience, so it is often good to begin with concrete examples, such as – in this chapter – a scenario with a ship where the philosophical problems always have a concrete reference for the children to test them out. Also, making the discussions about the children in some way can keep them engaged with the issues. Putting the insights and ideas about the ship into the context of their own lives and experience can bring it vividly to life for them. Many of the children will have been thinking along these lines already and it can be reassuring for those children to find that there is a whole tradition of thinking about these ideas stretching back many hundreds – in some cases thousands – of years.

End Box

See also chapter…on memory loss

Download The Ship of Theseus