Phil 101: Intro to Personal Identity (Part 1 of 2)

Clones and Perfect Duplicates

Suppose we grew a clone of you. Would the clone be the same person as you? Or would it be a different person?

Well, let’s say we raise the clone in a locked room, with no English newspapers or radio or internet access or anything like that. It’s attended by nurses who never speak English in its presence. It grows up to be 18 years old. Do you think it would be able to speak English?

No? Why not? It’s a copy of you, and you can speak English.

The answer is that clones are just genetically the same as their originals. They can grow up to have very different properties, because what properties you have isn’t just a function of your genes. Your environment also plays a role.

Do you know any genetic twins? They can have very different tastes and opinions. If they have different careers, and went to different schools, then we can expect them to know different things. On a given Monday afternoon, one twin might be thinking about a math problem, and the other one thinking about dinner. If we made a clone of you, the relationship between you and your clone would be just like the relation between genetic twins. (Except that in the clone case, the “twin” was born much later.)

As we talk about personal identity, we’ll sometimes be talking about perfect duplicates. These are supposed to be much more like each other than a clone would be like its original. They don’t just have the same genes. They are supposed to be molecule-for-molecule identical. So if the one duplicate is digesting a ham sandwich, then so too is the other. If the one duplicate has a chipped tooth, or a broken arm, or a migraine headache, then so too does the other duplicate. At a given moment, their brains would be perfect Xerox copies of each other.

(Of course, just because they are perfect duplicates at one moment t, doesn’t mean they will remain duplicates. Even in a deterministic universe, people who start out as duplicates can encounter different stimuli, and so gradually end up diverging. When we talk about having perfect duplicates, we just mean they are duplicates at a given moment t.)

Suppose we made a perfect duplicate of this sort of you. (Don’t ask me how we did it! Just suppose we did.) Would it be the same person as you?

Different Ways of Talking about “Being the Same”

We should distinguish:

  1. being one and the same thing as X — philosophers call this strict or numerical identity


  1. what philosophers call qualitative identity — identity in the sense of being perfectly similar, or exactly alike

Recall our example when we discussed Leibniz’s Law of Tamar who’s the mother of two children that she buys duplicate bikes for. The children have “the same bike” in the sense of having two bikes that are perfect copies / qualitatively identical. They have “the same mother” in the sense of one and the same person being mother to both of them. (This isn’t a difference about bikes and mothers; in other sorts of cases you may talk about “the same bike” where you also mean one and the same bike, not just another duplicate bike.)

Other examples: Clark Kent is numerically identical to Superman. Two pieces of chalk, or two Xerox copies of the same original, are not numerically identical. They are only qualitatively identical.

Recall Leibniz’s Law. It said:

If A is identical to B, then: every property that A has, B also has to have, and vice versa.

How should we understand this? Was it talking about numerical identity or qualitative identity?

It’s not talking about qualitative identity. For we generally count things as qualitatively identical even if they don’t share all of their properties. For instance, two pieces of chalk fresh out of the box may be qualitatively identical, but the one piece of chalk will be held in my left hand, and the other won’t be. That is a respect in which they don’t have the same properties.

To make sense of this, we need to introduce a further distinction, between:

  1. how a thing is in itself, or intrinsically


  1. how the thing is in relation to other things (how it is relationally or extrinsically)

Being 70 inches tall is an intrinsic feature of me. Being taller than Joe is not an intrinsic feature of me. It doesn’t just depend on how I am in myself. It also depends on how tall Joe is. Similarly, being 10 feet away from Joe is not an intrinsic feature of me, either.

When philosophers talk about “qualitative identity,” that isn’t understood to include being identical in all extrinsic or relational respects, too. The objects will usually differ in some of their relations to other things, like whether they are held in my hand, or how far away they are from the window. To be qualitatively identical, things only have to share their intrinsic features.

The contrast between intrinsic and extrinsic properties is a very important distinction in philosophy. We’ll be coming back to it several times this term.

Anyway, so we see that A and B can be qualitatively identical — they can share all their intrinsic properties — even if they differ in respect of some of their extrinsic or relational properties. If A and B are one and the same thing, though — if they are numerically identical — then it’s hard to see how they could differ in respect of any of their properties. (Here we set aside properties having to do with what people believe or doubt or hope about A and B.)

So we should understand Leibniz’s Law as saying:

If A is one and the same thing as B (that is, A is numerically identical to B) then: every property that A has, (whether intrinsic or relational), B also has to have, and vice versa.

Does Leibniz’s Law Imply That It’s Impossible for Things To Change?

Suppose in 2010 we spend a pleasant afternoon sitting underneath a little tree — call it Junior. In 2030 we return to the same location and find a much taller, fuller tree — call it Senior. People living nearby tell us that no tree was ever cut down or removed from that spot. Does Leibniz’s Law allow us to say that Junior and Senior are one and the same tree? It looks like the trees have different properties. Junior was little, but Senior is very tall and full. Since they don’t have the same properties, how can they be one and the same tree?

Intuitively, we want to say that Junior is the same tree as Senior, even though Senior is much taller and fuller than Junior. That is, Senior is NOW much taller and fuller than Junior WAS. Perhaps this is important. After all, can’t we say:

So we have to pay attention to the time at which things have various properties. Senior isn’t just tall, period. He’s tall in 2030. But it also seems true to say that Junior is tall in 2030. There was a time when Junior wasn’t tall. That was back in 2010. But Senior wasn’t tall then, either.

So Junior and Senior do have all the same properties, after all — if we pay attention to the time at which they have them. Neither of them has the property of being tall in 2010. Both of them have the property of being tall in 2030. So Leibniz’s Law permits us to say that Junior and Senior are one and the same tree, after all.

If we like, we can build this reference to time into Leibniz’s Law, as follows:

If A is one and the same thing as B, then: for every time t, if A exists and has certain properties at t, (whether intrinsic or relational), then B also has to exist and have those properties at t, and vice versa.

As we proceed into our discussion of personal identity, it will be crucial for you to keep these points firmly in mind:


To illustrate once more: The clock on the wall right now is numerically identical to the clock that was on the wall five minutes ago, even though some of its properties have changed (its hands have moved).

A thing’s identity conditions are the facts about that thing in virtue of which it is one and the same object over time, and a different object from its neighbors. When we come back after twenty years and look at the grown-up tree Senior, what makes it one and the same tree as the tree we sat under back in 2010? What makes it a different tree from the trees in the neighbors’ yards (which may be qualitatively very similar to it)? To answer questions like that is to say what the tree’s identity conditions are.

What Is Personal Identity?

Questions about personal identity are questions about what the identity conditions for persons are. Questions about when we have a case of one and the same person, and when we don’t.

You are one and the same person as the five-year-old who broke your mother’s vase. (Of course, you are qualitatively different from that five-year-old; but you are numerically identical to him or her.) What makes you the same person as that five year old? What makes you a different person from the guy sitting next to you? To answer these questions is to say what your identity conditions are. That is the topic of the next section of this course.

One way of figuring out what your identity conditions are is to ask what sorts of changes could you “survive”? What sorts of changes would be ones where it’s still you who is left around afterwards, and what changes would bring your existence to an end?

This is another technical notion. It’s not built into the meaning of “surviving” in this sense that something still be (or ever was) alive. As we’ll see later in the course, some philosophers and many ordinary people think that when your body dies, you still exist, only now as a dead body/corpse. On these views, being alive is not an essential property of you, and dying is a change you can “survive” — in this technical philosopher’s sense.

For the next few weeks, we’ll be looking at some more radical kinds of changes, and asking whether these are changes you could survive: whether you would still be around after these changes took place.

sam brown, explodingdog

For instance, recall the thought experiment I mentioned at the start of term:

Ship of Theseus

A useful way to begin tackling questions of personal identity is by considering the Ship of Theseus. This is a famous philosophical example, dating back to ancient times. Theseus was an Athenian hero who sailed to Crete, defeated the Minotaur, rescued some Athenian captives, and sailed them back to Athens. In his honor, the Athenians preserved the ship, and used to sail it around on parade once a year. Over time, however, various bits and pieces of the ship needed repairing. No problem. They just found some new parts, shaped just like the old parts and made out of the same materials, and made the repairs. It seems plausible that it was still one and the same ship, even after some repairs of this sort were made. This is just the ordinary sort of maintenance that all well-kept ships experience.

Now suppose that over time, every part of the ship eventually gets replaced in this gradual, piecemeal way. Is it still one and the same ship? Here it seems a bit more problematic than before to say it’s the same ship. But we might persuade ourselves that, since the replacements were all gradual and piecemeal — and since it doesn’t appear like any one of the replacements caused the original ship to cease to exist, and a new ship to spring up in its place — that yes, this is still the same ship as the Theseus’ original ship. Of course, it has undergone some qualitative changes. But that was also true of the ship when Theseus still owned it. Back when Theseus owned the ship, it was constantly getting wet, having pitch applied to it, and also having parts repaired and replaced. That’s ordinary life for a ship.

Now let’s add a new twist to the story. Suppose that one enterprising Athenian collects all the discarded parts of the original ship. As time goes by, his collection accumulates, until eventually he has all the pieces of the original ship. Now he carefully puts these pieces together, according to the design of the original ship. Next time the annual “Ship of Theseus Parade” comes around, he brings out his ship and bills it as the real Ship of Theseus! The other Athenians are dismayed. They thought that the ship they had been carefully repairing all of these years was the Ship of Theseus.

Who is right? Which ship is one and the same as the original ship? The carefully repaired ship? Or the ship built out of all the discarded pieces?

This puzzle raises a number of interesting issues about the identity of ships and other human artifacts over time. Can an object survive the replacement of some of its parts? Can it survive the gradual, piecemeal replacement of all of its parts? Can it survive being disassembled and rebuilt according to its original design? (As the ship built out of the discarded pieces purports to have done.) And so on. Some of these questions go beyond what we can pursue in this class. But other questions raised by the Ship of Theseus puzzle have counterparts for persons.

Epistemological vs Metaphysical Questions

When thinking about the Ship of Theseus, and about identity conditions for persons, it is important to keep epistemological questions separate from metaphysical questions. The metaphysical question here is: What would make this one and the same ship (or person) over time? The epistemological question is: How can we tell whether or not this is the same ship (or person)?

These questions are related. In particular, the way we answer the metaphysical question will make a big difference to how we should answer the epistemological question. Nonetheless, they are different questions, and it is important not to confuse them. For instance, suppose that we decide to answer the metaphysical question by saying that, if the new ship is built out of all the same planks and pieces as the original ship, then they are one and the same ship — even if the ship has been disassembled in the meantime. In that case, we would still be left with the question, How can we tell whether these are the same ship? How do we go about determining whether or not the new ship is built out of all the same planks and pieces as the original ship?

Similarly, when we’re talking about whether it would be possible to “store” your personality on tape, and then revive you in a new body, or as a sentient computer, we also need to distinguish the metaphysical questions:

Could such-and-such an individual be the same person as you? What would make it the case that he or she was, or was not, the same person as you?

from the epistemological questions:

Who would be able to tell? How would they know?

If some of the crucial evidence gets lost, it might be that some future person is the same person as you, even though no one (including you) is in a position to know this.

As with the Ship of Theseus, here too the metaphysical questions and the epistemological questions are closely related. The way one answers the metaphysical question will make a big difference to how we should answer the epistemological question. Nonetheless, they are different questions, and it is important not to confuse them.