The Problem of Identity Through Time


The ship of Theseus provides a classic example of the problem associated with holding on to the notion of an identity that persists through time. The ship gradually undergoes replacement of its constituent planks until by the end none of the original planks remain. Does it then retain the same ‘identity’ as the original ship? To further complicate the picture, all the replaced planks are reassembled into a ship ‘identical’ to the original ship, with the same design. The problem then is the stark contradiction in asserting that the replaced ship and the newly reassembled ship are the same. An Aristotelian resolution of this problem would be to assert that the replaced ship is the ‘same’ ship as the original ship owned by Theseus. The argument rests upon the preservation of the essence (design) despite the accidental changes (replacement) of the constituent planks. However according to the same argument the reassembled ship should be identical to the original ship implying that it should be the same as the replaced ship, which clearly cannot be the case. Therefore the contradiction seems to persist. I argue against the Aristotelian resolution of the problem and claim that a Humean analysis of the problem not only suggests the problematic nature of holding onto a persistent identity but also provides a possible resolution by rejecting the idea of a persistent identity.

Before delving into the heart of the contradiction, it is important to realize the need to clearly define certain important terminologies that appear throughout such a discussion. These terms, identical, same and identity are noted in the above paragraph in quotation marks. However I will not define these terms just yet. Rather than providing a general definition, I will derive them from the respective philosophical positions in question and at the end provide an explanation as to why the definitions upheld by Aristotelian position eventually lead to the contradiction. Nevertheless I will make the reasonable assumption that identity is transitive:

1)    A and B have the same identity

2)    B and C have the same identity

=>  A and C have the same identity

For the purposes of this discussion the ship of Theseus sets sail every year and on returning it has a plank replaced by a new one. Year after year, eventually all the planks are replaced and the removed planks are then reassembled into a new ship with the original design. The annual replacement of a plank ensures that the process is gradual, which I argue is an important criterion for such a notion of identity to persist. To make this obvious, imagine replacing all the planks of the ship within a day, it is definitely more difficult to retain this notion of a persistent identity and to call it the same ship.

Now, from an Aristotelian standpoint things can undergo either essential or accidental changes. Essential changes alter the ‘essence’ or in our case the design of the ship. An accident change does not alter the design, in our case replacing the planks that constitute the ship. So in the Aristotelian sense to say that despite the accidental changes, the ship remains the same suggests that the definition of identity coincides with the definition of essence. As essence persists through these changes (accidental) so does the identity of the ship. Therefore at the end even though all the planks have been replaced, the ship still retains its identity or the essence or the design (shape, size). So to say two things are same in this sense is not to assert that they are indistinguishable but that they have the same identity (essence). Identical on the other hand refers to the case where they not only share identity (essence) but also the accidental properties (the planks) and are indistinguishable. This then suggests that the reassembled ship is identical to the original ship, as it not only has the same planks but also the same shape and design and therefore the essential properties. And by the transitivity of identity, the replaced and the reassembled ship would have to be the same (share identities) leading to the contradiction that it sought to avoid. However a possible means of avoiding this contradiction would be to reject the premise that the reassembled ship has the same essential properties (therefore the identity) as the original ship. This would then avoid the conclusion that it has the same essential properties as the replaced ship. However this would imply that there is more to essential properties than the design, the shape, and the size of the ship. Whatever the reassembled ship lacks, it does not seem to be sensible to the senses otherwise it could be fixed and made identical. It has to be a property inaccessible to the senses that the original ship had and retained through the replacement of its planks. However this seems too easy a resolution. I argue that positing a property inaccessible to the senses doesn’t add anything to the ship. By the virtue of its inaccessibility to our senses, we have no means of determining either its existence or non-existence. What then allows us to deny the existence of this property in the reassembled ship? So the best the Aristotelian standpoint could do is to assert that if that property (essential) is absent in the reassembled ship, it wouldn’t be identical to the original ship. But we have no means of determining whether this is true and therefore it does not provide a conclusive resolution to the contradiction.

This is exactly what David Hume would have rejected. In his A Treatise of Human Nature, Hume argues against the idea of an identity (or as he calls it the substance) that inheres the properties since we have no means of discovering it. According to him, things are no more than the collection of their properties. And therefore there is no persistent identity that survives through the changes. I argue that the Humean notion of identity avoids the contradiction mentioned above and provides a possible resolution. According to the Humean standpoint, the replaced and the reassembled ships are clearly not the same as they are made up of different planks. This clearly eliminates the contradiction. However this seems to lead to a different contradiction. Since there is no identity that persists through the changes, the replaced ship cannot be the same as the original ship; share the same identity.

Before tackling this counterargument it is important to redefine the notions of identity, same and identical. Unlike in the Aristotelian framework we can no longer talk about a persistent essence as the source of identity. In the Humean sense there is no persistent identity. So then what is an identity? I argue that the way to include the notion of an identity into such a position is to propose a dynamic identity. Identity in this sense is nothing but the mental label given to a collection of properties. It is dynamic in the sense that it evolves with the changes to the properties. Therefore an object that undergoes the change would no longer be the same (share identities) with the object before the change. This then implies that when I replace a plank of that ship, its identity changes. Therefore identity is no longer understood as independent and inherent of the properties but as a product of their union.

The above argument suggests that it was a wrong question to begin with to ask whether the replaced and the original ships are the same. There is no inherent identity that possesses the properties that persists through the changes. Identity as we understand it evolves and is dependent on the union of the properties that make up a thing. This I argue is the only sustainable notion of identity (if we want to persist with one) that avoids the contradiction.

However there seems to be a problem with this resolution. Despite its ability to not only resolve but also to redefine the problem, it is disturbing to assert that there is no persistent identity on intuitive grounds. It is quite clear that on changing a plank we perceive the ship as the same as the one before (the replacement). But our intuitions are by no means the definite check for accuracy. And the reasons why we have these intuitions are neither significant for the purposes of this paper nor within its scope. However, the fact that these changes are gradual might be important for this notion of a persistent identity. And the fact that the changes are small and often insignificant in comparison to the portion of the object that remains unchanged. In our example changing a plank a year is not only gradual but the change itself is almost negligible compared to the hundreds of planks that remain unchanged. On returning after the second year, our notion of the identity of the ship has now evolved to incorporate the replaced ship. So it is no longer equivalent of replacing two planks on the original ship but replacing one on this ship with the changed notion of identity. By the time all the planks have been replaced the ship shares none of the constituent materials as the original ship but since we have clung on to this idea of a persistent identity that remains the same despite the changes to the ship’s constituents, we find the need to posit the existence of such an identity. In other words, such an identity can be seen as a mechanism of resolving the discrepancy in our perception of the ship. And this is exactly what the Aristotelian explanation of the problem seems to do. It has this idea of essence that persists through the accidental changes, which we call the identity. But that as shown before is not an adequate an explanation to resolve the contradiction.

It is clear that the idea of a persistent identity as defended from the Aristotelian position fails to adequately account for the contradiction posed in the problem of the ship of Theseus. And despite the fact that it runs contrary to our intuitions, the Humean rejection of a persistent identity not only helps resolve the contradiction but also drastically redefines the problem.



  1. Hume, David, A Treatise of Human Nature, (New York: Dover, 2003 edition)
  2. Gallois, Andre, “Identity Over Time”, The Stanford Encyclopedia of Philosophy (Summer 2012 Edition), Edward N. Zalta (ed.), URL = <>.
  3. Cohen, S. Marc, “Aristotle’s Metaphysics”, The Stanford Encyclopedia of Philosophy (Summer 2012 Edition), Edward N. Zalta (ed.), URL = <>.
  4. Image source:, last accessed: Dec, 15, 2012.





Tenzin Rabga studied Physics at Massachusetts Institute of Technology. His interest lies in Philosophy of Science, Physics, and Ethical theory. In his free time he enjoys reading books and discussing philosophy.
  1. Identity and Individuality:

    The structure and the behavior of things contribute to their individual being and function. In case of a ship, the identity involves its basic function; the thing must float on the surface of water. Its substance doesn’t change when you replace it with the same kind of original substance that contributes to the function of flotation. In case of man, the ‘essence’(what you are) is determined by what you do. Man’s existence precedes the fact of his essence. Man must understand what it is to be a substance and what it is to exist. From the time of conception, man’s physical identity changes all the time during all the stages of his existence. While the identity is changing, the man is still recognized as a specific individual because of the unchanging nature of his individuality.

    • Tenzin Rabga Reply

      Thank you very much for your comment. I think you raise the same sort of questions that were raised in the article. To try and understand your points in the terms used in the article, it seems like what you refer to as the individual seems to coincide with the idea of the persistent identity. And you make a very interesting point in asserting that ‘man’s existence precedes the fact of his existence’. I think this is something Hume as an empiricist would agree with. And therefore I think from the Humean point of view it is difficult to assert for a persistent identity independent of the characteristics of an individual that verify its’ existence; the empirical imprints it leaves upon the senses of some other individual with the sensory faculties to detect them. Hence it is not as much the idea of an identity or the ‘idea of an individual’ per se that is problematic, but the assertion of an identity independent and inherent of these properties; which seems to suggest the precedence of the essence over the existence of a person, contrary to what you point out should be the case.

  2. My understanding of man is mostly derived from Biological or Medical Science. Since human life begins at conception as a single, fertilized cell, the human person at that stage is aware of the fact and condition of its existence. As the cell divides and develops into a very complex system, the perception of identity develops and changes. At all stages, the individual who exists, is aware of the fact, and condition called existence. In the external world, an observer uses various criteria to describe the identity of the individual that is observed. But, within the complex organism, there is structural, and functional organization that help to recognize the existence of the entity called ‘self’ and distinguish it from non-self. For example, if I place before you two samples of blood and ask you to identify it, your visual experience would not be of help or use. Whereas the body without using any visual information can recognize the self from non-self. Man comes into existence with the same genome which changes his morphological appearance all the time and yet it retains the ability to recognize its own self as an unchanging, specific individual. Man may describe his essence(what you are) using his imagination, and thoughts/feelings, but that essence would have relevance if and only if he maintains his existence.

    • Tenzin Rabga Reply

      Your assertion on the importance of recognizing existence and this distinction between the self and the other resonates with Descartes’ famous quote, ‘I think therefore I am’. First of all I think it’s important to make a case for the distinction in the comparison of ships to the comparison of human beings. When we talk about the human existence we run into the mind body question that has undoubtedly troubled thinkers across time and space. As for the ships, it is clear that all we can talk about is the material constituents and the functional properties that this ensemble seems to have. Talking strictly about such objects confined to the realms of material constituency, any identity or essence it assumes is provided to it by the contemplating mind of a human person. But as far as detecting the presence of that material object (the ship in our case) is concerned, the best that the person can do is to collect a myriad of sensory stimuli that he/she can gather and frame a coherent notion for that collection and impose on to it a persistent identity. This according to the arguments provided in the paper is inconsistent as we reorganize and replace the material constituents of the ship. The key for this seemingly consistent notion of an object with an intact identity to survive is for these changes to be both gradual and insignificant compared to the entirety of its’ composition. Otherwise the contradiction is stark. As for the human person and the idea of an identity, Hume as an empiricist takes his reasons to the logical extreme and suggest the same arguments argue against the notion of a persistent soul independent of the sensory properties of that body. However a more thorough treatment can be found in the Buddhist concept of selflessness a logical consequence of the philosophy of emptiness. As much as our self-investigation can take us, we can only see our selves as a composition of our sensory and mental/conscious activities. The self that at the surface seems very solid and obvious does not survive the continuous subjection to logical investigation; it cannot be separated as an entity independent of the body and mind ensemble. The philosophy of emptiness, staying true to its’ teachers in the middle way school also objects against nihilism; denying the existence of the self at any level whatsoever. Instead it suggests that at the level of the conventional truth we do need this ‘label’ of self to have a consistent picture of the world and ourselves but at the level of the ultimate truth, when put to the ultimate logical test, it doesn’t survive independent of the aggregates that assume this label. So Descartes’ existential assertion and your point seems to coincide with the Buddhist idea of the conventional truth.

  3. FUNDAMENTAL DUALISM: While speaking about ‘things’, we have to recognize the problem of fundamental dualism; the separation of living, and non-living things. There is a flaw in the suggestion that animate, and inanimate things could be compared. A ‘thing’ is said to have 1. matter, and 2. form. The matter is described as the potentiality of the thing, and the form is the actuality of the thing. Living things also have matter and form. The problem is that of the distinction between living matter, and non-living matter. Both are not the same. The problem of identity could not be the same. Plato discussed about ‘forms’ and discussed about the form of cat. I would be happy if you separate the categories; ships do not belong to the same category as men. The identity of a boat can be simply changed by changing the flag it may fly, or it can be dismantled and be converted into a carriage. We have a saying in my native language as to how time and place can change the identity of ‘things’. A carriage is used on land for transportation, and when it reaches water, it can be dismantled and be assembled to perform the function of a boat used for water transportation. I like the theory of ‘Sunyata’ or ‘Emptiness’ that was formulated by Acharya Nagarjuna as I belong to the same region(Andhra Pradesh) and the caste community(Brahman). I would be happy to discuss the concept of Nothingness and Emptiness. The problem is that of coming to an understanding about the terms that we use in our conversation. If you define the terms like, body, mind, soul, consciousness, and truth, I would be able to explain my understanding of those terms. We may not be able to remove the differences in our understanding of terms. At least, we will know as to what the other person is saying. It may appear that my views are in agreement with the views of Rene Descartes. It is not really so. I have explained my understanding about the mind, body connection in my blog post. If time permits, please review the same.

    • Tenzin Rabga Reply

      I completely agree with your first point which unfortunately comes across as a misunderstanding of what I might have stated in my previous comment. I agree with the cartesian dualistic approach to animate and non animate objects. What I meant to state in the previous comment is that, it is for this reason we should differentiate the two categories and compare the ships to ships and humans to humans. As for the more important of the points you raise in your response, let me clarify certain terms that run the risk of adding unnecessary ambiguity to the discussion. ‘BODY’ as far as I can fathom is the physical constituent of a human person that is accessible to the senses. ‘MIND’ is more edgy a term to define clearly. To keep the discussion open for a flexible interpretation, I would be happy to comply with a definition that acknowledges the mind as the conscious aspect that transcends its’ physical basis. For example, the smell of a rose originates in a physical basis but the sensation or the consciousness (awareness) associated with that stimulus would be a mental phenomenon. So in other words mind would be heavily tied to consciousness and memory. This is as close to a definition of the mind as I can come to. ‘SOUL’ on the other hand in my usage refers to the persistent identity that is not only independent of the physical basis (body) but also transcends and survives after the death of the body. CONSCIOUSNESS is again a tricky term to define. I would be willing to define consciousness as something as simple as awareness; Awareness of the surrounding and the self through sensory stimuli or subtler mental processes that do not have as substantive a physical basis like thinking and imagining. I use the term TRUTH in accordance with the Buddhist understanding of the ‘reality’ another similarly ambiguous term. To help you understand what I mean, I suggest you read any standard Buddhist text that propounds on the two truths (conventional and ultimate). Any such understanding of the truth(s) should coincide with what I meant by the specific use of the term truth in the previous response. As for the views you put forth in your blogpost, u say that ‘Soul has to be defined as the manifestation of the consciousness’. And you say ‘The Consciousness, the Awareness of individual cells in multicellular organism functions to achieve the Functional Unity of the Whole Organism’. It is quite clear that this awareness at the cellular level relies heavily on the external stimuli that each individual cell receives and acts accordingly in the multicellular ensemble. So inevitably the soul is dependent upon the constituent cells, which are physical entities. This understanding of the soul is contrary to mine and therefore this might be where the misunderstanding stems from, if at all. And you differentiate yourself from Descarte’s proposition by stating ‘I am consciousness, therefore I am’, and it is clear why you do so, since according to you, the soul is a manifestation of the consciousness of all the cells and not independent of it. But to sum up and what I believe would be a Humean thing to say is that such a manifestation (soul) is in no ways more obvious to the empirical senses than Descartes’ I; I have no empirical means of determining which one is more accurate a description. So to claim for the existence of such a manifestation based on the recognition of existence of the human person, seems in no way more valid than the mere recognition of Descartes’ I. Moreover I find it problematic when you attempt to say anything more about this mere recognition at all. I think the recognition is all you can afford, and to say anything more and validate any other claims you would need to have an empirical basis.

  4. I am glad that you understand the problems involved in discussing such simple issues like identity. In the past, philosophy included a variety of subjects apart from science. Slowly, religion moved away from philosophy and has developed into Theology. Many concepts shared by Theology are believed without further discussion. Due to rapid expansion of information, Science has to be organized into a variety of disciplines and mostly people focus their attention to narrow fields of study. There is no general agreement on several terms like life, consciousness, and others. We could have a meaningful conversation if we discuss this topic using a simple organism like Amoeba proteus. Man is too complex and people had expressed too many views and people tend to have subjective opinions. Take a little time to read up on this simple protozoan known as Amoeba and see if we can understand the terms like consciousness, soul, and identity. Let me see if we can clarify our views using a simpler model of life. I use understanding based upon Cell and Molecular Biology and hence the information that I share is based upon empirical Science, and has been experimentally verified.

  5. The Identity of ‘The Living Tibetan Spirits’ : In the context of the discussion about Identity, I would like to invite my Tibetan friends to ask me about the Tibetan Spirits that inhabit my consciousness.

    • Tenzin Reply

      R. Rudra Narasimham, how you have not only inductively argued with such a realistic approach rather than academically prove something objectively. It has blown me seriously.

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