Author: Chad Vance
Category: Metaphysics, Historical Philosophy
Word Count: 998
That table you’re sitting at; or that coffee mug in front of you; or that phone: have you seen these objects before?
Presumably you have. But, consider some of the assumptions behind this: first, that there are such things as tables and mugs, and, second, that these things continue to exist through time (e.g., from yesterday to today).
What is required in order for such material objects like these to survive through time?[1] This essay explores that question, by way of a famous story about the Ship of Theseus.
1. The Ship of Theseus
Imagine this:
A man named Theseus sails a wooden ship. (Call it ‘Original’.) Every week, he arrives at port and has one plank of his ship replaced. After many years, so many replacements have been made that finally Theseus’s ship (now call it ‘Repaired’) no longer has any of its original parts.[2]
Is the ship at the end of the story the same ship as the one at the beginning? That is, are Original and Repaired the same ship?
2. The Repaired Ship
Let’s start with a simpler question: consider the ship after just a single part—one board—is replaced. Is that still the same ship? It seems so. After all, the following principle is overwhelmingly plausible:
Gradual Replacement: if an object, A, is composed of many parts, and has a single part replaced, resulting in object B, then A and B are the same object.
This is just a fancy way of saying that composite objects survive the replacement of a single part.
Imagine what would follow if we were to reject this principle: a mechanic replaces the spark plugs on your car. As they hand you your keys, you complain, “That isn’t the car that I arrived in today. Dude, where’s my car!?” Indeed, even the table, the mug, the phone: these would not in fact be objects that you have ever seen before—material objects are constantly exchanging atoms and electrons with their environment, and thus having their parts repeatedly replaced at the atomic scale. How absurd!
Yet, if we accept this principle (as it seems that we should), then it immediately follows that Repaired is the same ship as Original. Consider: if Original, the ship with zero parts replaced (call it S0) survives the replacement of a single part, then that ship (the one with a single part replaced; call it S1) also survives the replacement of a single part. (That is, S1 survives as S2, the ship with two parts replaced.) And then S2 survives as S3; and S3 as S4; and so on, since each of these changes is merely an instance of replacing a single part of a ship that has many parts. In short, the Gradual Replacement Principle entails that it remains the same ship throughout each and every one of these replacements.[3] In other words, it never ceases to be Original. That seems implausible.[4]
3. The Reconstructed Ship
Now consider the following addition to our story: [5]
Unbeknownst to Theseus, every time he has a part of his ship replaced, the ship repairperson sets the old part aside with the intent of eventually constructing a ship of their own. At the end of Theseus’s story, just after Theseus receives Repaired (the ship with all new parts), the repairperson sets sail on a ship which has all of Original’s parts, in the exact same arrangement: call this ship ‘Classic’.
If the repairperson voyaged around declaring that it was he who sailed the Original ship of Theseus, would he be correct? Classic really does seem to have a legitimate claim to being Original. After all, the following principle is overwhelmingly plausible:
Same Parts: if object A (at time-1) and object B (at time-2) are composed of all and only exactly the same parts, arranged in exactly the same way, then A and B are one and the same object.
This is just a fancy way of saying that objects always survive whenever their exact composition survives. Put even more simply: same parts, arranged in the same way, same object.[6]
Imagine what would follow if we were to reject this principle: imagine my friend is convinced that his bicycle is not the same one that he parked there earlier. You carefully verify that it has the exact same parts in the exact same places as the bike he parked earlier and share this information with him. Your friend, if he rejected the Same Parts Principle, would claim that this isn’t enough to convince him that it’s the same bicycle and would demand some further information.[7] But that seems absurd!
4. A Paradox
If we accept the Same Parts Principle (as it seems that we should), then Classic is Original, since they have exactly the same material composition. But, wait: earlier, we had already discovered that Repaired is also Original. Yet, it can’t be the case that Classic is Original and Repaired is Original; for then it would follow that Classic and Repaired are the same ship as each other! This is entailed by the following principle:
Transitivity of Identity: if A is identical to B, and A is identical to C, then B is identical to C.[8]
It would be absurd to reject this principle: it’s part of what it means for things to be identical.
Yet, now we appear to have a predicament. It is overwhelmingly plausible that objects survive the replacement of a single part. So, Repaired is Original. And it is overwhelmingly plausible that objects survive so long as their composition survives. So, Classic is Original. But it is also overwhelmingly plausible that identity is transitive, in which case Repaired and Classic are one and the same ship—i.e., they are just one ship and not two. But that is absurd! There are clearly two ships here, not one!
5. Conclusion
We have just discovered that, together, three overwhelmingly plausible principles entail something impossible. So, we must reject one of them! Which should it be, and why?
Notes
[1] For the related question of when material parts compose an object in the first place, see Objects and their Parts: The Problem of Material Composition by Jeremy Skrzypek.
For the related question of what is required for persons to persist through time, see Personal Identity: How We Exist Over Time by Chad Vance, and Psychological Approaches to Personal Identity: Do Memories and Consciousness Make Us Who We Are? and Are We Animals? Animalism and Personal Identity both by Kristin Seemuth Whaley.
[2] This story is originally found in “The Life of Theseus” (c. 75 A.D.) by the ancient Greek historian, Plutarch.
[3] For discussion of how objects can persist despite changes in their material parts, see Form and Matter: Hylomorphism by Jeremy Skrzypek.
[4] This reasoning resembles the following: “If someone with a full head of hair loses one hair, they are not bald. If they lose one more hair, they are not bald . . . and if they lose one more hair, they are not bald . . . so someone who has lost all their hair is not bald,” which is clearly false. For discussion of this type of reasoning, and its potential implications for these ship cases, see Vagueness by Darren Hibbs.
[5] This addition to Theseus’s story is initially found in the works of philosopher Thomas Hobbes. See Thomas White’s De Mundo Examined (1642; ch. 12, parts 3-4) and De Corpore (1655; part 2, ch. 11).
[6] After all, if the repair person had used all of the old planks from Theseus’s ship to construct a giant wooden horse statue, we would agree that the horse statue is not the same object as Original.
[7] For example, perhaps it would not be the same bicycle if a friend of yours had dismantled it and put it back together while you were away. Perhaps dismantling an object and putting it back together causes a thing to cease to exist. (After all, this appears to be all that has happened in the intervening time between Original and Classic.)
[8] This type of principle is often appealed to by psychological theories of personal identity: see Psychological Approaches to Personal Identity: Do Memories and Consciousness Make Us Who We Are? by Kristin Seemuth Whaley.
References
Hobbes, Thomas (1642). Thomas White’s De Mundo Examined (ch. 12, parts 3-4).
Hobbes, Thomas (1655). De Corpore (part 2, ch. 11).
Plutarch (75 AD). “The Life of Theseus.”
For Further Reading
Gallois, Andre (2026). “Identity Over Time”, The Stanford Encyclopedia of Philosophy, ed. Edward N. Zalta.
Locke, John (1690). “On Identity and Diversity” (II.xxvii), An Essay Concerning Human Understanding.
Wasserman, Ryan (2013). “Material Constitution”, The Stanford Encyclopedia of Philosophy, ed. Edward N. Zalta.
Related Essays
Personal Identity: How We Exist Over Time by Chad Vance
Psychological Approaches to Personal Identity: Do Memories and Consciousness Make Us Who We Are? by Kristin Seemuth Whaley
Are We Animals? Animalism and Personal Identity by Kristin Seemuth Whaley
Vagueness by Darren Hibbs
Form and Matter: Hylomorphism by Jeremy Skrzypek
Objects and their Parts: The Problem of Material Composition by Jeremy Skrzypek
About the Author
Chad Vance is a Teaching Professor of Philosophy at William and Mary. His specialties include the metaphysics of modality and environmental ethics. He earned his PhD in philosophy from the University of Colorado Boulder. rintintin.colorado.edu/~vancecd/index.html
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