How to teach the ship of Theseus

Why this matters

A middle schooler can hear the ship of Theseus and answer in seconds: the ship becomes a new ship once most of its planks are new. On a worksheet that answer looks finished, and nobody asks what one plank changed between plank 50 and plank 51. The student never tests the rule, and the teacher has to wait for the class discussion to find out who is still counting parts.

The ship of Theseus paradox

Plutarch writes that the Athenians kept the thirty-oared ship Theseus sailed home in. "They took away the old timbers from time to time, and put new and sound ones in their places," and philosophers argued over it, "some declaring that it remained the same, others that it was not the same vessel" (Plutarch, Life of Theseus 23.1). Thomas Hobbes added a twist: a custodian collects the original planks and puts them back together, which leaves two ships and the question of which one is the original (Stanford Encyclopedia of Philosophy).

Britannica calls the ship of Theseus "a paradox regarding identity over time." No single replaced part seems to end the ship, yet the ship at the end is "materially completely different" from the first one (Britannica). The Stanford Encyclopedia's entry on identity over time sets out several competing answers to the ship of Theseus thought experiment, so a student's answer is judged by the reason behind it.

In the Flint activity, each student says at which plank the ship stopped being the same ship, looking at a chart Sparky draws of old and new planks. Unlike a worksheet, Sparky answers that student's own rule: a cutoff like "half the planks" goes back to the chart, and a reason the student can defend gets Hobbes's two ships.

Sparky gets Ben to test his own rule

Ben ends up dropping "most of the parts" for a rule about care over time, then defends it against Hobbes's second ship. Every student gets the same story and charts, and Sparky's questions depend on the answers each student gives.

Sparky uses Ben's bikeSparky sends Ben back to the chartA reason Ben can defend gets the harder Hobbes case

B

Ben

Interest: Restoring an old bike with his dadStarting point: Thinks identity is just "most of the parts"

Ben

hi im ben. me and my dad are fixing up an old bike from the 80s that we found in my grandpas garage. we put new tires on it and a new chain and new brakes so far

Sparky

The ship had 100 planks. Every year, one old, rotting plank was replaced with a brand-new one. Is it still the same ship? Here's my question for you, Ben: at which plank do you think it stops being the same ship — and why?

Ben

i think its when half the planks are new. like at plank 51 there are more new parts than old parts so its basically a new ship. same with my bike, once most of the parts are new its not grandpas bike anymore

Sparky

I like that you applied the same rule to your bike. But look back at the chart for a second. What actually changed between plank 50 and plank 51? It was just... one plank. Can one single plank really make the whole difference?

Ben

no thats kinda weird. one plank is so small. if i swap one more part on my bike tomorrow it doesnt turn into a different bike overnight. so maybe its still the same ship the whole time because they changed it a little at a time and it kept being theseus ship, like people kept taking care of it. we're fixing grandpas bike, not making a new one

Sparky

That's really solid reasoning, Ben. Now here's where it gets trickier. While the Athenians were replacing planks one by one, someone was secretly collecting all the old discarded planks. After 100 years, they used those original planks to rebuild the ship exactly as it was. Which one is the real Ship of Theseus, and why?

Ben

ok this one is harder. i still say ship A because it never stopped being used and taken care of, ship B was just a pile of old wood for 100 years. but its weird bc ship B has all the real original planks

Sparky

Your reason is clear — continuity of care is what makes it the same ship for you, not the original materials. Here's one last push to test that rule: if your grandpa's bike had been left in the garage completely untouched for 40 years, and you and your dad are now fixing it up with new parts, is it still your grandpa's bike?

Dropped the half-the-parts cutoff, then defended his rule against Hobbes's two ships

Ben is an example student, and Sparky's replies are shortened. Next, Ben said the bike's original frame and its story matter, so Ship B has a point, and Sparky asked whether that rule still works for a ship with no original plank left.

What Flint did for Ben

• Sparky drew a chart of the ship's old and new planks, with plank 50 and plank 51 marked, before asking Ben where it stopped being the same ship.
• When Ben said the ship turns new at half its planks, Sparky sent him back to the chart to ask what one plank changed, without telling him he was wrong.
• After Ben gave a reason he could defend, Sparky drew Hobbes's two ships, then tested his rule on his grandpa's bike.

A teacher can't do this by hand for 30 students, and Sparky does it in every student's chat.

The activity page shows who is still counting parts

While the class works, Needs attention shows the teacher which student is stuck and on which message. After three or more students submit, activity analytics lists what the class reasoned well, where students went wrong, and a follow-up to practice next.

Set up the activity for free

Open the ship of Theseus activity in Flint's public library, select Use activity to copy it into a free Flint account, and assign it to a class. Sparky draws the plank chart and Hobbes's two ships because the activity's guidelines tell it to, so there is no setting to turn on. Students type their answers, and each chart opens next to the chat.

For more ethics activities, see Flint for ethics.