Are we the same physical person today that we were yesterday?

How do we know we are the same person if most of our cells, except for the brain, have been replaced?

That philosophical thought experiment is known as the Ship of Theseus (or Theseus's Paradox).

The Core Paradox

The experiment was first recorded by the Greek historian and philosopher Plutarch in the late 1st century. It describes a ship used by the legendary hero Theseus that the people of Athens preserved for centuries. As the wooden planks decayed, they were replaced one by one with new, stronger timber.

Eventually, every single original piece had been replaced. This raised two central questions for philosophers:

1. Is it still the same ship? If it is, then an object can exist independently of its original material.

2. When did it stop being the "original"? If you replace one plank, it's clearly the same ship. But if you replace 100%, is it still the same?

The Hobbesian Twist

Later, the philosopher Thomas Hobbes added a second layer to the puzzle: Imagine someone collected all the old, rotting planks as they were removed and reassembled them into a second ship.

• Which one is the "true" Ship of Theseus?

• The one in the harbor made of new wood, or the one in the dry-dock made of the original (but old) wood?

Connection to Modern Life

This isn't just a debate about ancient boats; it’s a fundamental question about identity. For example:

• Human Biology: Most of the cells in your body are replaced every 7–10 years. Are you still the same person you were a decade ago?

• Technology: If you upgrade every component of your computer over five years, is it still the same PC?

You might see a parallel in legacy codebases. If you refactor a 40-year-old COBOL system into Python line by line until no original code remains, is it the same "system" or a brand new one?

You may wish to explore how different philosophical schools (like Aristotelianism or Four-dimensionalism) attempt to solve this.