Identity

Is an apple today the same apple tomorrow?

Identity and belonging have intrigued me since childhood, and this question has been with me for a long time. This post is where it finally took shape. Identity is one of the first principles of life: so much of what we understand about people, things and the systems we build rests on knowing what makes something the same as itself.

What is identity?

Let’s call things and beings alike “entities”. In a philosophical sense, identity is a relation each entity bears only to itself. In a sociological sense, it is the set of attributes that make an entity unique. Either way, it is the fact of being the same as itself.

That is a stricter idea than the everyday one. Two copies of the same book are “identical” in ordinary language: they share every attribute, but they are still two books. Philosophers call that qualitative identity. I mean numerical identity: one book being the same book as itself.

Identity and persistence

So is the apple today the same as the apple tomorrow? If an entity is the same as itself, does it need to be the same across time? Even if nothing else changes, time does. Yet entities seem to keep their identity through it: identity persists. And that persistence is exactly what makes identity hard. Under what conditions does an entity persist through time as one and the same entity?

Composition as identity

Composition is the relation between a complex entity and the parts it is made of. One could argue that the apple today is made of the same parts as yesterday, and so it is the same apple. An entity is often thought of as nothing over and above its parts. But is identity really defined by what something is made of?

Biologically and chemically, the apple has changed, even if we can’t see it. In a changing world, everything is changing, all the time, and things still keep their identity. So identity must survive some changes in what a thing is made of.

Take ourselves. We feel like one continuous self, but our bodies keep replacing their cells. Red blood cells live for about four months. Some white blood cells live for days, others for years. Skin cells last about two or three weeks, the cells lining the colon about four days, while most brain cells last a lifetime. At any moment, we are made of different parts from the ones we were made of before. Yet we are still ourselves.

The paradox of identity

So what does it mean for an entity to be the same if it changes over time? If every part is replaced, as in the old paradox of the Ship of Theseus, in what way is it the same?

Say your grandfather gave you a bicycle (A). With regular use and natural wear and tear, you replace one of its parts. Is the bicycle you have now still the one your grandfather gave you? Since identity survives some changes in composition, yes, it is the same.

Over time, you replace each part, one by one, as it wears out. Eventually the bicycle you have (B) has not a single part left of the original (A). Is it still the bicycle your grandfather gave you (A == B)? By the same logic, yes.

Now say your neighbour collects all the discarded parts and builds a bicycle of his own (C), made of precisely the parts that made up the original (A). Which one is the bicycle your grandfather gave you? Is A == B, or A == C? Since all the parts of C are the parts of A, one could argue that A == C.

But if A == B and B != C, then A != C. Yet every part of A is a part of C, and every part of C is a part of A. So A and C are two different bicycles even though their parts are the same, while A and B share no parts at all, and are the same bicycle.

A

The bicycle your grandfather gave you.

B

The same bicycle, every part replaced one at a time.

C

Your neighbour’s bicycle, built from A’s old parts.

A = B? by continuity, yesA = C? by its parts, yesB = C? no

Continuity in space and time

So what does it mean for an entity to be the same as itself when its parts change? One answer is continuity: to persist, an entity must trace a continuous path through space and time. That path allows parts to change, as long as the change is gradual and the form of the entity is preserved. By that rule, B is your grandfather’s bicycle, because it never stopped being a bicycle along the way.

But say that, instead of replacing the parts of bicycle A, you took it apart and stored the parts in separate boxes. A few days later, you took them out and put the bicycle back together (B). There was no bicycle-shaped object tracing a continuous path in between. Is A == B? It is made of exactly the same parts, put together the same way, so surely it is the same bicycle. So continuity alone can’t settle it either.

There may be another answer, one that lies less in the bicycle and more in the person looking at it: perception. But that is for another day.

So what does it mean for an entity to be the same?