There's an ancient Greek legend about a ship that launched from the port of Theseus and sailed around the world. Probably not around the whole world...just around the world. During its journey, the ship encountered many dangers. Storms ravaged its sails, which had to be replaced; the floorboards gradually gave way and had to be changed out one by one. Eventually, crew members decided that they liked the look of some island or another along the way and stayed behind, and as the ship sailed on, more and more of it and its crew was replaced until finally, not a single original plank of wood or hand on deck remained.

Now, here's the question. When the ship finally returned to Theseus, was it the same ship that left? The answer, whether we can find it or not, lies in the concept of identity. Philosophers describe identity as the relation that a thing bears only to itself, so whatever makes a thing uniquely what it is defines its identity. And if two things are identical, they're said to share an identity relation. Now whether two things are the same might seem blindingly obvious, but of course it isn't, because philosophy.

The philosophy of identity can get pretty slippery, largely because like the ship of Theseus, things change, and when they do, they eventually stop being what they are and become something else. This goes not just for mythological ships, but for all kinds of things. Like, this mug could acquire a new identity, and this money, and you, and even...Batman.

(INTRO)

So, riddle me this: is Batman identical to Bruce Wayne? And when I say identical, I don't mean that they just look alike. I mean that they share the same identity - they're literally one and the same. So you might think the answer is yes, Bruce Wayne and Batman are identical because everyone knows that Bruce Wayne is the man behind the cowl. Not only that, you might say Bruce Wayne is the very essence of Batman. He saw his parents get killed in that alley. He became the Dark Knight. No one else could take his place.

But hold up. People have taken his place. It turns out that Dick Grayson, the original Robin, has pinch-hit for Batman, donning the cape and cowl to protect the streets of Gotham in his boss's absence, and so has police commissioner Jim Gordon: after Bruce Wayne was reported dead, he reluctantly accepted the role himself. Both of those guys have been Batman. So if the person inside the costume doesn't make Batman "Batman," then what does?

Seventeenth-century German philosopher Gottfried Wilhelm Leibniz came up with a principle that might help solve the puzzle of who Batman is. He called it the indiscernibility of identicals. The idea is that if any two things are identical, then they must share all the same properties. If Leibniz is right, then the ship of Theseus became a new ship as soon as that first plank of wood was replaced. As soon as its parts were not all original, then the ship suddenly acquired a new property, and with a new property came a new identity. So likewise, Bruce Wayne and Batman can't be identical because they have different properties. Bruce Wayne, for example, has the property of being a millionaire playboy, but Batman doesn't. That guy is all business. Meanwhile, Batman has the property of having fought the Joker, but Bruce Wayne doesn't. And the different versions of Batman aren't identical to each other, either, because the person wearing the costume, even if it's the exact same costume, which it usually isn't, is going to have different combat techniques and even, say, a different moral code.

So is there a limit to how much something can change and still be the same thing? Let's head over to the Thought Bubble for some Flash Philosophy. Think about this puzzler, originally offered by contemporary American philosopher Allan Gibbard. A sculptor takes a lump of clay and names it "Lumpl." She then forms Lumpl into a statue, which she names Goliath. Are Lumpl and Goliath identical? Our intuition might say yes, because they're composed of exactly the same amount of the same physical stuff. It's been kneaded and molded into a different shape, but still, no part of Lumpl is not Goliath, and no part of Goliath is not Lumpl.

But consider this. If we wreck the statue and smush it into a big blob, Goliath is gone, because part of what it means to be Goliath is to be shaped like a statue. But Lumpl, being a lump, doesn't have a set shape, so it will remain after Goliath has been destroyed. So on one hand, Gibbard observed, Lumpl and Goliath seem to be identical, because they're the same exact thing, just in different shapes. But on the other hand, how can Lumpl and Goliath be identical if one can exist while the other does not?

Thanks, Thought Bubble. One way to try and make sense of identity and explain how an object endures over time is by making a distinction between what we call essential and accidental properties. Essential properties are the core elements needed for a thing to be the thing that it is. Accidental properties are traits that could be taken away from an object without making it a different thing. Think about a dog. A dog without a tail is still a dog. Give him a shave or a silly poodle 'do, don't let it bark, you still have a dog. So all those things are accidental properties and they're pretty easy to pick out.

But it can be pretty tough to find its essential properties, the things that if they were absent would make the dog not a dog anymore. And the more something changes, the harder it can be to determine its identity. A tree can lose its leaves and still be a tree, but if you cut the tree up and make it into a bunch of notebooks, is it still a tree? And if you think a notebook is not a tree, then at what point in the process did the tree lose its treeness? Was it when it was cut down and thus no longer living? Maybe, but isn't a dead tree still a tree? Or did it happen when the tree was cut into pieces and was lying on the ground? Did it make a difference when those pieces were collected? How about when they were ground up into pulp? The tree stopped being a tree when its essential property was lost, but when exactly that happened depends on your perspective.

Plus, many thinkers reject the concept of essential properties altogether. Existentialists, for example, deny the very existence of essential properties, and ancient Greek philosopher Heraclitus famously said that you can't step in the same river twice. What he meant was, nothing is identical to itself because everything - including both you and the river - is changing all the time. So not only is the ship of Theseus a new ship by the time it gets back to Theseus, but it was a new ship the first time anything about it changed.

Now, it's amusing enough to think about whether ships and trees endure over time or what makes Batman Batman, but ultimately, why should you care? How does this affect your life? Well, if objects are important to you, then you'll want to know whether you have the same object that you think you have. And when it comes to tangible stuff, we tend to value persistent identity. If you take $20 out of my wallet and buy lunch and then stop by an ATM and replace it, is the money that you gave me identical to what I had this morning? If Nick breaks my favorite coffee mug here and replaces it with a new one and I can't tell the difference, do I still have the same mug that I started with? What if your dog runs away while I'm house-sitting for you and I replace your dog with a new dog that is so similar you can't tell? Is it the same dog? What's the difference between the money and the cups and the dogs? Philosophers actually have a word that explains why we think one $20 bill is the same as another but one corgi isn't the same as her cosmetically identical sister. It's fungibility, the property of being interchangeable with other objects of the same kind. Most people think that money is fungible, because it's just a place-holder for the value it represents - and the value is what we really care about. As long as there's a $20 bill in my wallet to buy me some nice Thai drunken noodles, I really don't care whether it's the same one I put there or not, though I would like to know what you're doing digging around in my wallet.

Now the coffee mug - this one's interesting. It seems like what matters here is why that mug that Nick broke was my favorite. If I loved it because it's the perfect size and shape to fit in my hands, and it keeps my coffee warm, then I'd probably be just as happy with a new one. So in that case, the mug appears to be fungible. But if I love the mug for personal reasons - like, say it was the first Crash Course Philosophy mug ever manufactured, or it was a gift from my dad - then a new one, even if it's cosmetically the same, wouldn't mean the same thing to me. Because it might be that what I care about isn't the mug at all, but some kind of abstract idea behind the mug, like my love for Crash Course or the connection between my dad and me. In any case, I've gotta ask: just, Nick, be more careful when you're around my stuff.

Today, we learned about different ways of understanding identity, including the indiscernibility of identicals and essential and accidental properties. We thought about how change does and doesn't affect identity and what it means for a thing to persist over time. And next time, we're gonna take what we've learned about identity and apply it to personal identity, thinking about what connects this guy to me.

(OUTRO)