Existence and identity are deep topics. One is reminded of Descartes’ immortal declaration of “I think, therefore I am.” In a way, that’s where we are headed in this chapter. The concept of personal identity makes you confront how you choose to define it, and also forces you to think about how you choose to define yourself.

We start with the thought experiment of Theseus’ Ship, which asks you to evaluate at which point a ship is still that same ship if you continually remove its components, and even more so if you remove the components and then replace them with entirely different ones. So it forces you to consider what you use to construct an identity. Is it just the individual components? Is there something about a ship that can be more than the sum of its parts? And what about us—what makes us a particular person, and if removed from our physical body, what would we then become?

From questions of what we can know as humans and an attempt to measure infinity, we can move on to an equally lofty topic that has similarly engrossed man for ages: who are we and why are we here?

In this chapter we’ll look at those thought experiments that confront the big questions of what constitutes a person, what it means to say that someone is alive and has consciousness, and what our identity is as human beings. What is a thing or person’s essence, the one aspect that, if changed, would make them no longer what they are? Are people’s identities real, fixed phenomena or are they more like social constructs or artefacts of language (a common outcome for many thought experiments)?

It’s one of those questions that seems simple on the surface but can be quite profound when you dig deeper: who are we really, and what would it take for us to not be that anymore? Perhaps in answering questions of who and what we are, mankind is indirectly asking what we are, and ought to do, i.e. what our purpose and function is in life. In the same way we cast our minds out to take in the full possibilities of time and space, we turn inward and try to understand ourselves and our fellow humans.

Plank by Plank

The thought experiment called “Theseus’ ship” is a well-known and poignant place to start off our enquiries. Theseus has a ship. For reasons only he understands, he gradually takes it apart day by day, piece by piece. One day, he removes one wooden board. Is the ship still the same ship after he removes this board? Most people would probably say yes. But if he continues carefully removing pieces, at what point would you say that it wasn’t the original ship anymore?

Carrying the story further, imagine that instead of simply removing pieces from the ship, Theseus removed parts and then replaced them with different parts. Is the ship still the same ship, or something different? If so, at what point exactly does it become a different ship? Finally, imagine that instead of just removing parts, Theseus took the individual pieces and reassembled them elsewhere. Where is Theseus’ original ship now? Is it the old one or the new one? Neither? At one point did it change over?

Here we see the result of the classic Greek atomistic mindset turned on human identity itself. We are invited to imagine that we ourselves are the ship. In a (gruesome) parallel, at what point would you lose your identity if a psychopathic murderer decided to whittle you down to nothing, removing parts of you piece by piece? (Unlike the ship, there would be the extra conundrum of whether you’d consider yourself to maintain your same identity after surely dying… but this is another path of questioning.)

Theseus’ ship has all the hallmarks of a great thought experiment: it’s simple but runs deep. What can it teach us about not just ourselves (i.e. the topic under investigation), but also the way in which we seek to answer these questions (indirectly also teaching us about ourselves!)? This thought experiment is great because it allows you to outline a set of possible answers, and examine the implications of each.

Firstly, you could potentially answer that the very moment you take a piece off the ship, you change it fundamentally, and it becomes a different ship. But if you wanted to say this, you’d have to concede that small atoms and molecules are constantly shifting into and out of the whole, with the result that the ship (or person) is never actually a static being with a fixed identity.

To the ancient Greeks, this was a ludicrous outcome and meant to be seen as paradoxical, but this viewpoint (“a man never steps in the same river twice”) is not inconsistent with what we know about the world. We are reminded of the Eleatic school’s single Being moving and shifting constantly—not a mathematical entity but an organic, organismic one.

A second potential answer is that the ship is more an idea than just the individual parts that make it up, and if you remove or replace parts, it’s still the same ship. This leads you somewhat down the path of Plato’s Forms (i.e. things are not merely objects in the material world, but abstract concepts, of which the physical is merely an instance).

Even if you replaced every wooden piece with an aluminium piece, say, the ship would still be the same ship. This view seems to hold that identity is more about a convention or deliberate attempt to group a collection of parts into a whole—it’s not merely the existence of the parts, but the idea behind them, the organizing principle, structure, form or blueprint. If we demolish you completely but rebuild you using the code in your DNA, it’s still you.

Others might say that identity is a question of continuity over time, and that the ship is whatever we’ve come to understand the ship as over time. The parts can change, the form can change, but if we keep on interacting with the ship as the ship, it is the ship, however it alters over time.

How can we use this thought experiment to improve or illuminate our own thinking about our own identities? Well, we can use it as a diagnostic—we all have beliefs and conceptions about ourselves and our place in the world, but they’re often completely unexamined, habitual, or merely inherited from family or the culture we live in. Take the time now to consider your own perspective. With Theseus’ ship in mind, answer the question:

• What makes you you? What part, when removed, makes you cease to be you?
• Is it your physical traits (your race, sex, age, physical features, species)?
• Is it your DNA, the idea of you as a whole?
• Is it the history you’ve already lived with that identity?
• Is it something somehow inside or because of your traits and structure, something like a soul, a personality, a string of thoughts (one level deeper than a blueprint)?
• Or perhaps is there no such thing as a ship, no such thing as a person, and the boundaries we’ve decided exist between separate structures are really for convenience, but don’t actually describe the world as it is?

Companies, musical groups, families, sports teams and the like can all change their constituent parts, yet still retain the same identity. All of us would say that an acorn, a fully grown oak tree and a pile of oak timber all share an identity, albeit at different life stages. As this example indicates, time is an important consideration. And even though most people would agree that cutting their hair doesn’t drastically change who they are as a person, they almost certainly will say that they are a different person than they were ten or twenty years ago.

This thought experiment doesn’t confront us with paradoxes so much as invite us to consider our working beliefs and mental models, and explore the implications they have. It teaches us to be explicit and deliberate in the thoughts we have, looking at what we believe rather than just believing it out of habit or convention. Exploring alternative answers (even counterintuitive ones) gives us a chance to more thoroughly evaluate the realm of possibilities—and perhaps adopt a mental perspective that is more accurate or more useful to us.

Follow this train of thought and you may find yourself benefitting in a spiritual, behavioral or psychological sense. For example, if each of your atoms and cells turns over many times every number of years, what’s to say you can’t be a completely different person after that time has elapsed? That would be inspiring, from a personal development standpoint.

On the other hand, if you are you no matter what, you literally could lose all your limbs, get total amnesia and undergo a full blood transfusion, and still you would not be able to shift your permanent identity.

Perhaps you only believe that some traits are essential (like race or species) whereas others are more malleable (like sex or gender—look closely at any debate around transgendered people and you will see clear echoes of Theseus’ core argument, on both sides). All potential perspectives here are valid; the question is, which one will best serve your purposes?

Every day, you may go around the world with an implicit understanding of who you are as a human being. It’s not explicit—you never deliberately and consciously decided to think this way, it merely happened, and continues to happen so long as you never question or examine it.

The issue is, your conception of who you are dramatically influences every aspect of your life—the only difference is whether you’re doing it consciously or unconsciously. If you never give yourself the chance to question and revamp any unhelpful beliefs and attitudes, you’re doomed to live with them and the limitations they bring.