Imagine you build a wooden boat. As time goes on, you find that part of the boat needs to be repaired, and you replace it with another piece of wood. This process continues until each of the original parts has been replaced by another. Now the question: is this the same boat? If not, at what point did it become a different boat?
This problem has significant implications for our view of the human mind. Each of the body's cells (and indeed, each physical part) is constantly replaced by new ones. Yet, it is surely the case that I am the same person today I was three months ago. Does this suggest an immaterial aspect of humanity?