Ship of Theseus is a thought experiment in which every piece of a ship kept in a harbor is replaced one at a time. The questions are: would the end result be the same ship or a new ship? If it is not the same ship; at what point did it stop being the ship of Theseus? If it is the same ship; would reassembling the removed pieces to form another ship result in another ship of Theseus? Surely the ship would not be in two places at once? What philosophies argue that the result is the same ship? And how do the philosophies defend their position that replacement doesn't affect or change identity?

Rephrasing the thought experiment to improve relevance: our bodies grow and change. The old, worn out cells are replaced one at a time. The atoms that make up our cells and bodies too are replaced. Is the end result the same human from before or a new human? What philosophies argue that the result is the same human? And how do the philosophies defend their position that replacement doesn't affect or change identity?


5 Answers 5


The Ship of Theseus is one of the more illuminating thought problems in philosophy, and it having been around for something like 2500 years indicates how insightful the early Greek philosophers were.

What the thought problem shows is that if one takes a reductive approach to identity, where identity is taken as identicality down to the reduced elements of a structure, then NOTHING in our physical universe, except perhaps for some elementary particles, are identical over time. This presents a major problem for reasoning, as classical reasoning is only valid if A ≡ A, and this thought problem shows that A ≢ A.

One major approach to identity taken based on this A ≢ A problem, is to accept that there IS no valid identity in our world. That objects in our world are heaps, composites, without any essence. Therefore identity is only an approximate state, used for pragmatic purposes, and logic, while often giving useful answers, COULD be invalid when applied to these soft-edged "A"s.

Hume was an articulate advocate of extending this "heap" or "bundle" theory of non-essence of things to selfhood. He pointed out that thoughts and experiences changed radically over a short time, and postulated that we are just a bundle of impressions. Selfhood today is generally identified more with memories, and personality inclinations, than with experiences, and these two are more stable than experiences, so Hume's radical variability moment/moment is not widely accepted today. But these other two also both change over time, hence his point that A ≢ A for selfhood is still very true. Hume lived in an era when logic was considered more absolute than it is today. And the Pragmatic alternative, where some ideas that are invalid in absolutist terms might still be "good enough approximations to get by" was still a few centuries off. So Hume followed the absolutist thinking of classical logic about selfhood -- if A ≢ A for selfhood, then in classical terms assuming "self" is a fallacy.

But in a more pragmatic worldview, a selfhood that is not reductively identical moment to moment, and is appreciably different year to year, could still be a useful item to identify and operate with in the world and with reasoning.

There are alternatives to the logical "objects are not real" and the pragmatic "both objects and reasoning are useful but not fully reliable". One of those is available to non-physicalists, where one can postulate an essence to an object, over and above its reductive elements. This essence could be part of an abstract realm such as with Plato's Forms, or could be a feature of consciousness or spirit. An example of the latter is the dualist philosopher Richard Swinburne who postulates that selfhood has an essence, a "thisness" that is maintained over time, despite changes in personality/memory etc. Swinburne's essence is clearly a soul, although he does not describe it as such.

Another alternative is to dispense with the reductionism that Hume and The Ship of Theseus assume is what matters for identity. The philosophy of science today has accepted that there are emergent structures in our universe, and these emergent structures are functionally independent of their reduced components. One can then tie identity to functions that an object performs, rather than to its constituents. Once one does that, there would then still be a Ship of Theseus, and it would be the continually repaired vessel floating in the Thebes harbor. And a set of discard boards, reassembled -- would then NOT be the Ship of Theseus. The function that defined which was THE ship would then be the continuity of the repair crew activities. Per this approach, if the repair crew were to leave, and the ship boards were broken and scattered, then a new crew reassembling and repairing as best they could, would still not be able to make THE Ship of Theseus, as they would never be sure of the provenance of the components and design. But add in pragmatism, and Ship of Theseus V1.1 might be a good enough stand in.

Emergence and functionalism lead to pluralism in science. And pluralism of reference frames — leads to logic explosions. Therefore, a noteworthy outcome of this thought problem is that for all three of the bundle/heap/no-objects, functional/emergence/pluralist, and the pragmatic "identity is approximate" answers, classical logic cannot be relied upon in our world. The only ways that applying logic to things in our world would still be fully valid, would be if they have an essence -- and Plato's Forms and their equivalent are widely rejected today -- with the sole exception of possibly selfhood.

  • 1
    Thank you. One thing to consider is that elementary particles are considered 'identical' in the sense of 'indistinguishable': able to be substituted one for one with no change being visible. Reasoning requires that things be explained in terms of other sorts of things, so an explanation of variation can only rest on a platform of sameness. Reductive explanation forces us to conclude that there are no unique things. Otherwise, all explanations would have to be infinitely long.
    – Scott Rowe
    Apr 23 at 13:37
  • Yes, elementary particles are the only exception to objects in our world NOT being reductively identical. At least we think not. The partial entanglement that sometimes persists between them has a possible logic effect on this that I have not fully parsed through, as a possible caveat. our declaration that reasoning requires reduction is in opposition to the consensus view of philosophy of science today, which is pluralist. See SEP Scientific Reduction, section 5. The pluralism of logic also makes any assertions about "logic must" invalid.
    – Dcleve
    Apr 23 at 17:17
  • @ScottRowe Here is a link for logical pluralism: cambridge.org/core/journals/think/article/abs/…
    – Dcleve
    Apr 23 at 17:17
  • 1
    @ScottRowe "'identical' in the sense of 'indistinguishable'" means to me having no identity. I have a (separate) identity because I can be distinguished even from my ("identical" ;-) ) twin brother, and because (hello, Descartes) I know I'm not him. Elementary particles have no identity in this sense, at least not after they have come close enough in space and time to each other. Apr 23 at 20:22
  • @Peter: Indeed, it has been semi-seriously proposed that there's only one electron, although this usually isn't taken too seriously because there are several physics-related objections to it (for example, electrons greatly outnumber positrons, which contradicts this hypothesis because positrons are equivalent to electrons moving backwards in time, and similarly we can identify scenarios where an electron worldline forms a closed loop, or appears to).
    – Kevin
    Apr 23 at 20:35

It comes down to definitions of words. If we say that the ship is the collection of atoms that it’s made of then replacing Aunty part of it means that the ship is divided into different parts that we can identify and track. Once all the parts have been replaced, whatever we have left is no longer in any way the original ship. On the other hand, if we say that the ship is an abstract idea embodied in a physical thing then replacing parts of the ohms thing doesn’t degrade the abstract idea and so the ship remains regardless of the replacements. So when you ask such a question you should specify how you choose to recognise a ship.

  • Right. But it's easier to just say that there are no ships. And, it's a great conversation starter.
    – Scott Rowe
    Apr 23 at 13:39
  • A great conversation starter but perhaps not a great world view if you want to achieve anything, especially in terms of marine transport
    – Frog
    Apr 23 at 23:15
  • "Aunty" and "ohms"? Very strange auto-correct fails.
    – Barmar
    Apr 24 at 2:18
  • Where would I go?
    – Scott Rowe
    Apr 24 at 2:46

Here is an answer from Thomistic philosophy.

All things have matter and form but God who is actus purus, pure form.

The wood constitutes the matter. The ship constitutes the form. A change in matter without destruction of the matter consitutes no change in substance. The ship is still a ship--it doesn't matter if it is made of wood or metal.

If we are talking about some sort of "spirit" which the ship has, i.e. the Ship of Theseus as a unique vessel or substance, then whether or not it remains the same vessel depends on the definition of its own substance considered in itself, and not merely as a ship.

If we are asking about the substance of the Ship of Theseus, and not whether it remains a ship after a change in matter (which we are not), then iff the Ship of Theseus [SoT] must be made of Cyprus wood, and iff the SoT must be made of only the first planks it was made of and no other (including any defects that are introduced), then merely applying a graft to, say, a hole produced by a cannonball, is sufficient to make it a different ship than the SoT and is not merely an accident because the substance is directly tied to the original construction completely such that no new construction can be considered part of it.

I think everyone would laugh if the three ships that sailed into the New World were considered to have their unique substance destroyed because of a bit of patchwork, so on this basis I would claim that if we define the unique substance to be based on origin such that we can say, "The SoT was made of this, but now is made of that," then the default answer I would give by this definition is, "no, it is still the same ship whose appearance only has changed over time, but not its substance".

For a more objective answer, what we conclude is that this is an incredibly subjective question. Without some objective substance to analyze or examine, it is impossible to determine what it is, so the best, most morally certain answer, really is, "inconclusive: insufficient data".

  • As an additional consideration: if the substance is defined by the name itself, "Ship of Theseus", then it has accidental being within the ship, so the substance would be destroyed by simply changing the name, e.g. "Ship of Harkinian", or by destroying the ship, e.g. burning it to ashes. This would also mean that any ship can become the Ship of Theseus by simply naming it that.
    – AMDG
    Apr 23 at 14:51
  • "We had to destroy the ship in order to save it." Apparently... The answer I would give would be in terms of a world line. The baby born long ago was me because at every moment since, there has been no divergence. This is the commonsense answer also supported by Physics. Not sure why it is still a subject of debate.
    – Scott Rowe
    Apr 24 at 2:43
  • @ScottRowe I don't quite follow. This is rather vague and appears unrelated to my answer.
    – AMDG
    Apr 24 at 15:57

Suppose Alfred owns a ship moored in the port of Honfleur in France. Suppose Alfred has enough money to pay for repairs to be carried out every day so that a bit of the ship is replaced every day and the ship is kept pristine all the time. Suppose every bit of the ship is replaced at one point or another within one year.

At the beginning of the year, Alfred owns the ship. At the end of the year, it is still Alfred who owns the ship.

Apparently, nobody really has any difficulty identifying the ship as Alfred's ship.

  • Pragmatically, logic works most of the time in our world, even though A ≢ A means it is subject to logic explosion. However, in exotic cases, the question of whether "that ship is Alfred's ship" does frequently come up, and leads sometimes to extensive legal disputes. For one of the more common such examples, if Alfred runs a ship chop shop, there will be LOTS of questions about whether that ship is really Alfred's.
    – Dcleve
    Apr 25 at 15:26
  • @Dcleve Sorry, I can't make sense of you comment. Logic explosion?! You sure?! May 11 at 17:27
  • Any contradiction in a logic system can theoretically be taken advantage of by an inventive arguer to assert the truth of either side of every claim. This is a problem for logic systems, and is called a logic explosion.
    – Dcleve
    May 11 at 17:57
  • @Dcleve 1. No, contradictions are just false and nothing therefore follows from any contradiction. - 2. What you mean is really called "the principle of explosion", not "logic explosion". The principle of explosion is just crap. May 11 at 18:05
  • You refer to both "Alfred" and "the ship" as items that last thru time. But those identifiers do not satisfy A=A. You use the rationale that they are close enough. Which is also true, most of the time, but not A=A, and is subject to logic contradictions as a result.
    – Dcleve
    May 11 at 19:01

"No man ever steps in the same river twice, for it's not the same river and he's not the same man." Heraclitus (apparently)

As @Frog suggests it is a matter of the meaning of words, and in this case what is meant by identity. If I talk about the river Wensum it has an identity that doesn't depend on the particular molecules of water that are flowing through it at any particular moment, but there is also a sense in which it has an identity as the state of the river Wensum at a particular moment. There is no great mystery here as long as we are clear what we mean by "ship" or "human".

Like the Wensum, the Ship of Theseus has an identity as the ship belonging to Theseus that remains the same throughout its life, regardless of how many components are changed. But that does not mean that the Ship of Theseus is remains identical (in a different sense of the word) throughout it's life.

With people it is more difficult as then identity can also mean your conscious identity, rather than just the person as an individual amongst others. This is not necessarily fundamentally encoded in the atoms, molecules or cells of your bodies, but in your "brain state", and arguably you might retain the same identity if that state were transferred to a simulation of your physical brain on an ultra-powerful computer (perhaps this has already happened ;o). This means the analogy with the Ship of Theseus a bit weak as there is an additional dimension to identity that is not present in a ship. Your body does not define your conscious identity in the same way that it defines your physical identity ("ah there's Dikran Marsupial"), which is the sense that seems to be being used with the Ship of Theseus.

So perhaps it is more a question of defining what we mean by "identity" (or at least being specific about which meaning we are talking about). Perhaps it is a matter of levels of abstraction. It may be that nothing is completely identical in an absolute sense, but they can be the same at a higher level of abstraction. The Wensum is the same river from one day to the next if we are talking about the river as a geographical feature, even if it isn't the same river in the sense of Heraclitus. This isn't a fundamental problem for logic or anything else AFAICS, it is just a problem with the ambiguity of natural language that we need to deal with.

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