Imagine a large wooden ship.
The type you would see in a movie complete with rowing chain gangs, a carven figurehead, and a huge sail. What would happen if a single plank of wood fell off of the ship and had to be replaced? The obvious answer is not much; the plank would be replaced and nautical life would go on. But what if, over the many years of the ship’s long life, every single piece of the ship from the oars to the masts to the floors, was replaced with an identical piece. Would the ship still be the same?
Let us complicate the question further. Imagine that when I removed parts of the boat, I did not throw them away. Instead I used them to build a boat identical to the one I asked you to imagine initially. We now have two boats docked side by side, identical in every way. The first (B1) is the “original” and has had every aspect of it completely replaced. The second (B2) is identical to B1 and in fact is made up of the original components of B1; however, it is a separate boat. We are therefore left to wonder, which boat, if any, is the original?
What is the purpose of this exercise? Why did I demand you conjure two imaginary boats? This thought experiment asks you to think about what identity is and when one thing becomes another. There isn’t a solid answer to this question. It is easy to view this thought experiment as an exercise in change, but it is truly a question of identity and what makes an object an object. A country, a business, a ship, a person — all of these undergo change over time and though the names of these things change, can we say the same about the object itself?
It is not just static objects that we must question; objects in flux also suffer identity crises. A candle that’s lit will melt over time and its shape will change drastically. From its unlit state, to half melted, to a puddle of wax at all points it is the same candle. No matter what the physical form of the candle may be, we still call it the same candle. This argument may imply that objects have some intangible element that defines them but this is clearly not the case for all things. The ashes of a book are not a book just as the ashes of a person are not a living being. The example of the candle only carries over for certain objects; the realm of the physical which is irrelevant in the case of the candle is crucial in other cases.
This issue has practical applications as well; consider that all the cells in your body will be different in seven years. Do you become a different person if all your cells have changed? What about your memories which unlike the case of the ships, can remain even when the physical body changes. I’ll ask you to imagine one final scenario: a criminal has been in jail for a ten years when she suffers a blow to the head, completely removing her memories of the past. Is this individual who possesses neither the physical makeup nor the memories of the person who committed the crime still guilty? Should they be released or should they still be held accountable for what was done in the past? For over 2000 years this problem has haunted philosophers and this ship isn’t going down anytime soon.