Introduction to Philosophy



Identity- tracking bodies through space and time




Nice to be with you again.

At least, I think it's you, and I'm fairly certain it's me.

If we are the same, you and I, what has kept us so over these past 10 weeks?

What has stopped you gradually transmogrifying into a quite different group of people? What has stopped me turning into someone else?

These are not exactly the questions that now beckon us, but they are closely related. Like them, our questions have to do with identity. When I say that I think you are the same crowd of people, what do I mean? What is it for a crowd of people to stay the same crowd of people over a period of time?

When I say I think I'm the same person as benefited from your warm and intelligent and forgiving company 10 weeks ago, what do I mean? What is it for a person to remain the same?

These questions are intriguing I think because they seem really difficult to answer satisfactorily, but they can also be important. I can only be held responsible in a court of law say for things that were done by me. Somebody wielded the axe. But justice is not obviously served if you put away a person who can prove that they were not the person who did the deed.

Or would it?

Can you think of a story where the crime of one person is put right by someone else entirely undergoing the penalty laid down by the law? What about this?

Let me remind you anyway of another reason why questions of my identity are important and not just interesting.


Anything I can do to keep going?

Is there anything I can do to maintain my identity after death? Does it matter what happens to my body - can I safely donate my organs, or would this put an end to all chance of continuing or resuming my life, or at any rate existence? Is it any good getting myself frozen, or do I stop being a person once I have stopped all conscious awareness for a year or two?


Continuity of body

The first possibility I want to investigate is that it is my body which gives me my identity.

That's to say, one 'theory of personal identity' is that it is my body which makes me the same person now as I was twenty weeks ago - this body is the very same as the one that was before you then.

But if I say this I have another question to answer immediately: what is it for this body to be 'the same' as that one?

The Mereological thesis

What does identity of a body, as a physical object, consist in?

One contribution to our struggle to answer this question is called the mereological thesis (or mereological 'principle'). It isn't an answer, just a point, say its proponents, a point we should bear in mind in looking for an answer. The mereological thesis is this:

An object retains its identity only insofar as the bits and pieces that make it up retain their identity.

Let me explain.

People are sometimes a bit shocked to learn that their body which they think of as familiar and reasonably stable is in fact 'changing' all the time, and quite radically. The hair is one thing: we know we have to cut it quite frequently. It is growing all the time - new bits being added at the root. Layers of skin are all the while sloughing off, with new cells being generated to take their place. But what we are told in general is that almost all the parts of the body are under a regime of continuous renewal, with the result that if you compare say the atoms that made up the body ten years ago mostly they will be different now. Goodness knows where they will be, but scattered far and wide. And a lot of people react to this by concluding that if you think of yourself 10 years ago the body you had then was not the body you have now.

If you think this, the principle you seem to be applying is this: if you have a complex thing like a body - complex in the simple sense of being made up of parts - it will only retain its identity if the parts it is made up of retain theirs.

Tightened up a bit, this is 'mereological principle'. Here is the formulation of one writer:

"For any compound objects, x and y, x = y only if every part of x is a part of y, and every part of y is a part of x.

I.e., an object continues to exist (from time t1 to time t2) only if it is composed of all the same components at t2 as it was composed of at t1. Sameness of parts is a necessary condition of identity."

Cohen, Identity, Persistence, and the Ship of Theseus,

(Cohen offers here brilliantly concise and clear notes on the problem of identity.)

You could say this is what Heraclitus was thinking in 504 BC.

A word now about Heraclitus, a key player in this project.


An ancient writer (Diogenes Laertius) records that:-

"Heraclitus was exceptionally haughty and supercilious and ... eventually became a misanthrope who lived in the mountains and fed on grasses and plants."
Collinson, 50 Major Philosophers, London, 1987, Routledge, p.10.

(That's all right then.)

Heraclitus wrote Fragments, or at least it is fragments that have come down to us. The most well known Fragment is this:

You cannot step into the same river twice.

Heraclitus was living at the far horizon of Ancient Greek civilisation. Nearer to us are the huge figures of Aristotle and Plato and Plato's teacher Socrates. We only know about Socrates through the writings of Plato. And a bit similarly with Heraclitus: nothing he actually wrote has come down to us, but we know of him from the writings of others. It was Plato who told us about Heraclitus' talk about rivers, explaining:

"Heraclitus, you know, says that everything moves on and that nothing is at rest; and, comparing existing things to the flow of a river, he says that you could not step into the same river twice." (Plato, Cratylus 402A)

As reported by Plato, Heraclitus' thought seems to be this. If you think of a river as made up of lots a cubic metres of water you will have to say that it isn't made up of the same cubic metres of water from one moment to the next. It is forever flowing into the sea, while at the other end water is continuously passing into it from its tributaries. It is constantly changing its composition.

Heraclitus (as glossed by Plato) takes this to mean that it must be changing its identity from moment to moment. Once you have stepped into it once, and then clambered out again to refly, or whatever those people in waders do, hundreds of gallons have flowed through, the composition has changed, and it's a new river you step into ready to tease the poor salmon again.

Could you think of the world like this, as in constant comprehensive flux?

It would a radical shift in our thinking! Take the lovely Lune Valley for example, actually very liberally supplied with people in waders regularly stepping in and out of the water in the strange ritual of the salmon fisherperson: this valley alone, if the mereological Heraclitus is right, must be running through rivers at a fantastic rate…

But look, you may say. These people not only jump in and out, but they come back year after year. And what do they come back to? Why, to the lovely river Lune.

That is to say, you may well be among those people who have felt that you don't have to think very far to realize that the mereological thesis, as represented here by Heraclitus, must be wrong. It would make nonsense of so much of our ordinary way of talking and our ordinary way of thinking.

(Both actually in my view completely hopeless reasons, but there we are.)

There is a variation on the mereological position which we should notice. It doesn't help much, but still. It says that the identity of complex thing is only threatened if a whole slew of its components are changed. If one or just a few are lost or replaced the identity of the thing stays unaffected - but if the component-swapping goes beyond a certain point - well it's then that the identity of the thing is lost.

I want to explore this possibility, as well as the absolute mereological position by reminding you of or telling you about perhaps the most famous vignette in the whole of philosophy: the story of the ship of Theseus.

The Ship of Theseus

This story is brought to us not by Plato but by Plutarch, an author living within two hundred years of Christ's birth. Plutarch's work was called Lives, in which he presented biographies of great people, bringing out how he supposed their virtues and vices impacted on their careers. Plutarch didn't originate the story, which probably goes back to the time of Heraclitus, and there are different versions. But here is his:

"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."

So you have a situation that reminds you uncomfortably perhaps of what they do to Old Masters when they 'restore' them. If there is a patch of paint which has darkened, or discoloured in some other way, very gently scrape it off and put a bit of the 'right' colour in its place - the colour originally achieved by the painter him/her self.

The uncomfortable thing is: if the painting is generally discoloured and the tarting up is pretty general, have you really still got the old Master once the 'restoration' is complete?

The real thing
The mummified Jeremy Bentham, courtesy University of Pavia's suggestive Gallery of Objects

Museum people often get into this embarrassment, or worse. They will construct replicas from scratch - and put these into glass cases for you to wonder at, and explain only in the small print that it's not the real thing at all, which either doesn't exist any more, or sits safely, perhaps in an indecently decayed or imperfect state in some vault where it can't be seen. I believe these days the display of the philosopher Jeremy Bentham in the University of London is like that - it used to be his actual body, mummified in accordance with his instructions, but is now a replica. The real thing was falling apart rather dreadfully, and is kept as a decaying heap, more or less, in the basement. But there's a congenial replica of how the mummified body used to look - in its prime, as it were, on display in the old spot in the foyer of London University Senate House.

Jorvic in York is I think another case in point. After queuing for quite a long time you go down and down into what is dressed up as a sort of archaeological dig, where all sorts of artefacts are on display, and you get the impression that you are actually seeing the tools and bones and brooches and so on in the place where they were actually uncovered. But actually, no. The actual archaeological site where Roman York was encountered is some way away, and all you are encountering is a piece of museumological theatre…

Does this matter? I report that for me it does. As a young thing, I always got a tremendous buzz from the ancient Egyptian mummies on display in Leicester Museum, in those in themselves ancient days, but the thrill depended absolutely on the thought that they were the actual bodies that had walked the earth at such a distant time. If they had been replicas --I hope they weren't - they would not have been without interest but all the magic would have gone.

I would be interested to know if there is much agreement with me on this.

If we are thinking of historical objects - jewels, crowns, swords, mummies - does it matter to you if they are real or not?


Mask of Coyolxauhqui. Jadeite.
Thanks to Royal Academy of Arts and Peabody Museum of Archaeology and Ethnology, Harvard University. Anonymous Gift. Photo © The President and Fellows of Harvard College

I put this to the test myself in London a couple of years ago, looking in on the exhibition of Aztec art and culture in the Royal College of Art. Lots of wonderful stuff, including the paraphernalia of human sacrifice, which the Aztecs went in for as the highest honour you could bestow on an Aztecan, or Aztecan prisoner of war. So there in front of you is a stone altar, and with it the flint knife. The spiel tells you how the lucky person - lucky because destined for immediate transfer to a better place, without the toil of spending years of hard graft preparation in the underworld - of how the lucky person is held down on the slab and has the priest excise his or her heart.

Does it make any difference if the altar on display is the real thing? The actual slab of stone upon which real people were laid out and dispatched? Does it make any difference if the knife you are looking at - if the actual knife you are looking at - was actually used for actual sacrifices?

I can confirm that for me, it's just how it was with the mummies. It matters a lot. It's all the difference between watching a play and acting in one.

Where are we?

- to have any significance, the paint of a painting has to be the very same bits of paint the artist him or her self applied.

- to create its frisson, the blade of the knife has to be the very blade that entered the ribcages of the actual victims, the handle has to be the actual handle grasped by the priest.

- to be interesting, the museum display labelled 'The Ship of Theseus' has to consist of the components which made the ship up when Theseus himself sailed on it.

These thoughts I suppose lend support to the mereological thesis, that for A to be B at least B has to consist of the same components as A.

Even so we have already on the table arguments against the mereological thesis:

- that I have had inumerable bodies as its individual cells come and go and

- that there are inumerable River Lunes

Let me make this confused situation worse by taking the story of the Ship of Theseus a stage further: Part 2: Another Ship of Theseus.

Another Ship of Theseus

The Ship of Theseus: let's go back to that most famous vignette. The ship is a famous one, so you want to preserve it for posterity. But its fabric is rotting and every few years you need to cut out a bit that's threatening to fall apart and put some sound timber in its place.

But in this well-intentioned, painstaking, unremitting care of this so-significant artefact there comes a point when there's not a shred of the original planking left. It's all gone, all replaced at one time or another with fresh timber.

The question is: Has the ship gone too? Can it be the same ship after such a wholesale replacement of fabric, or have we inadvertently destroyed what we sought at such great cost to preserve?

What do you think?

Well, does it matter? Surely there is just a teaser here, not a serious problem at all. We can say it is the same ship or it isn't the same ship, the reality is clear and unproblematic. There is no mystery here. What is happening is totally straightforward. We are replacing over time all the ship's planks.

Maybe there's a hint of something a little serious at stake though if we develop the story a little.

Let's imagine that there are enterprising used timber merchants in play, enjoying a contract with the keepers of Theseus' ship. The used timber people, trading as Dell Bros, are entitled by their contract to take away any material that is discarded from the museum where Theseus' ship is on display. As the museum authorities find a plank in the old ship showing signs of decrepitude and take it out, taking it out the back while easing a spanking new length of weatherproofed timber in its place, Dell boy sends his bro to collect what has been discarded and take it away, as their contract allows, nay stipulates.

They do more than take the rotten and discarded planks away. They collect them.

And out of their collection they build - a ship.

Visiting the museum on a regular basis to keep track of exactly whereabouts on Theseus' ship each discarded plank must have come from, they are able to put them back together again, exactly as they were before. By the time all the planks have been replaced, the Del boy construction is complete.

It's not terribly seaworthy, but as Dell boy points out in their advert, as they open their newly completed exhibit to the public, it has every claim to be Theseus' actual ship. That is why, they explain, it is astonishing that their entrance fee is no more than the one charged by the outfit next door, which is after all offering the public no more than a replica.

If you had just £10 in your pocket, who would pay to see the ship as cared for by the museum, lovingly replacing each plank as necessary, and who would rather spend their money seeing the Dell Boy rebuild?

Here's argument to show that everybody here, though they may have voted differently have in fact voted exactly the same.

I We have good reason to say that the ship in the museum is the same as the same as the ship Theseus sailed in.

II We have good reason to think that the Dell boy rebuild is the ship Theseus sailed in.

III But if A is the same as B and

A is the same as C then

B must be the same as C


If we accept I and II we have the conclusion that the ship in the museum is the Dell Boy rebuild.

So it doesn't matter who you pay your £10 to. If you go to the museum you will see the museum ship, but since this is the same as the ship Dell Boy rebuilt you will be seeing that too.

And vice versa.

Spatio-temporal continuity

Why are we inclined to say the museum ship is the ship Theseus sailed in?

Perhaps: It traces a continuous path through space-time.

If you had kept the ship under constant surveillance you would have seen it maintain the same form continuously from Theseus sailing in it to its appearance in the museum many years later. Changes in the fabric will have happened, but no jumps in space-time, and no alteration in characetristic shape.

This is the criterion of spatio-temporal continuity.

The same lion?

Is this lion the same as this one?

You can look at this one - noting its characteristics and listing them in as much detail as you think appropriate.

I can then give you access to this one and you can repeat your scrutiny.

Suppose your descriptions are exactly the same.

Do you have a good basis there for judging whether your two descriptions are of one and the same lion?

Of course not!

Two lions can be impossible to tell apart from looking at the two of them separately.

What extra information do you need? What extra information would give you a result?

The obvious suggestion is: if you were allowed to keep the lion I show you under continuous observation.

That would enable you to tell conclusively whether what you were seeing was one lion twice or two lions once.

It would enable you to say whether this was the same lion as this.

The posh word for the feature you would be checking on in this way is spatio-temporal continuity.

We talked of the special frisson you get when you have before you a very ancient artifact - the Aztec sacrificial knife for example, or, for me anyway, the mummies in Leicester Museum.

Is it spatio-temporal continuity that is essential here?

Inadequacy of the principle of spatio-temporal continuity

Spatio-temporal continuity seems crucial in some cases then, but sometimes our ordinary way of talking flouts it.

If you are moving house and you flat-pack your wardrobe and it happens that the doors go in one van and the draws in another and the back goes in a third and the screws left out by accident go in the car, you can surely reassemble the thing in your new house. What you put together is surely the same wardrobe as the one you took to pieces a few days ago. Yet there is no spatio-continuity to 'the' wardrobe itself, only to the different bits.

The two tests fighting

So we have two considerations which seem to play a part in our concept of identity: the mereological thesis and the principle of spatio-temporal continuity.

And they sometimes fight.

When we wonder which ship is the real ship of Theseus, the Dell boy rebuild or the ship in the museum, we get into paradox because we seem to be applying two different criteria -

(a) Are the parts all the same? and

(b) Is there spatio-temporal continuity?

In this case these two tests give conflicting results.

Why are we inclined to say the museum ship is the ship Theseus sailed in?

Perhaps: It traces a continuous path through space-time.

Why are we inclined to say the Dell boy rebuild is the ship Theseus sailed in?

Perhaps: all the parts of the ship Theseus sailed in are in the rebuild.

Thus by the mereological principle the two could be the same.

So we look forward to the question of what makes for identity in a person confident in the knowledge that as far as continuity of physical objects are concerned we are totally confused.


I've been looking a long time for an excuse to play a bit from one of my favorite films, Terry Gilliam's Brazil.

It's very funny, that goes without saying, but it is also a very persuasive vision of the world we are edging into. One aspect of this is the way in which we are increasingly woven, we individual human beings, into a fabric which is mostly not us but technology, with information systems digitalising us, recording and archiving everything about us, and using that to deliver goods and services, yes, but also to maintain control.

Gilliam makes the technology wonderful but flawed, just I think like it is. You have electric sliding doors which are ace when they work, but they don't always work, and then they just stay shut, or just as hopeless, open. Or as Gilliam has it, automatic universal hot drink makers which of course go wrong, squirting hot water on your toast - toast which has been made automatically with brilliant efficiency for you but which gets just as soggy when hot water is squirted all over it as it would have done had you made it yourself.

Another thought Gilliam forces us to confront is how modern dominant governments need an ever present threat like terrorism to sustain their authority and necessity. This clip ends with a 'terrorist' bomb going off, the first of many in what is in my opinion Gilliam's masterpiece.

My excuse for showing it is very feeble, but has to do with continuing identity. As you watch it, you have no chance at all of working out what it is. I will just say this: watch Jonathan Price, the chief character, in his wonderful hat.



Woman in waders

Thanks to ADG International Marketing

Anything I can do to keep going?

Mosaic mask of Tezcatlipoca, Aztec. Human skull, mosaic of turquoise antd jet, eyes of shell and pyrite.
Thanks to the Royal Academy of Art and the British Museum, London. Photo © The British Museum, London

River Lune at Caton

Thanks to Lancaster Priory

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