Philosophy Quiz: Identity Thought Experiments

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Identity Thought ExperimentsQuestion 1 of 20

A teleporter works by scanning and destroying a person's brain, then creating a new brain with an identical structure in a new body, leaving the rest of the original body intact but brain-dead. The new person wakes up with all the original's memories.

This scenario creates the strongest tension between which two competing theories of personal identity?

Psychological continuity theory and soul theory.

Brain theory and animalism (the bodily criterion).

Functionalism and psychological continuity theory.

Soul theory and brain theory.

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Philosophy Quiz

Philosophy Quiz: Identity Thought Experiments

Practice Identity Thought Experiments in Philosophy with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

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Question 1

This scenario creates the strongest tension between which two competing theories of personal identity?

Psychological continuity theory and soul theory.
Brain theory and animalism (the bodily criterion). (correct answer)
Functionalism and psychological continuity theory.
Soul theory and brain theory.

Explanation: This variant isolates the brain from the rest of the body. Brain theory holds that personal identity follows the brain. In this case, the 'person' moves to the new body because that's where the brain's information (and thus its functional structure) is. Animalism, or the bodily criterion, holds that a person is identical to their physical organism (the whole animal). In this case, the original animal/organism remains behind, brain-dead. Therefore, the two theories give conflicting answers about where the person is. The person, according to brain theory, is on Mars. The person, according to animalism, is the brain-dead body on Earth. This creates a direct and powerful tension. The other pairings do not capture this specific conflict as precisely.

Question 2

The Ship of Theseus problem and the teleportation problem both probe the nature of identity over time. However, the teleportation problem introduces a critical element not typically central to the Ship of Theseus problem. What is this element?

The possibility of exact duplication. (correct answer)
The principle of spatiotemporal continuity.
The relevance of an object's function.
The composition of an object's constituent parts.

Explanation: While the Ship of Theseus involves two potential candidates for an identity (the repaired ship and the reassembled ship), the teleportation problem, especially in its 'fission' variation, raises the issue of perfect, simultaneous duplication. This creates a logical problem for identity (which must be a one-to-one relation) that is more acute than the Ship of Theseus problem, which involves a temporal process of change and reassembly. The other options are central to both problems: spatiotemporal continuity (B), function (C), and constituent parts (D) are all criteria considered in analyzing the Ship of Theseus.

Question 3

Consider a case where the Ship of Theseus is not repaired but is instead placed in a machine that, over time, replaces every single atom of the ship with an atom of a different isotope of the same element (e.g., Carbon-12 is replaced with Carbon-14). The ship's structure, appearance, and function are utterly unchanged. This scenario is problematic for which theory of identity?

Functionalism, because the ship's function is perfectly preserved.
Spatiotemporal continuity theory, because the ship remains in one continuous path.
A strict mereological essentialism, which claims identity is tied to the original constituent matter. (correct answer)
A narrative theory, as the ship's history and name remain continuous.

Explanation: Mereological essentialism is the view that for an object to persist, it must retain its original parts. Some versions are stricter than others. A strict version holding that identity is tied to the original matter (the specific atoms) would be directly challenged here. Despite no discernible change in form, function, or location, the ship is materially entirely new. Therefore, a strict mereological essentialist would have to say it is a different ship. The other theories are all supported, not challenged, by this scenario: function is preserved (A), spatiotemporal continuity is maintained (B), and its narrative is unbroken (D).

Question 4

A philosopher argues, regarding the Ship of Theseus, that the term 'the Ship of Theseus' is merely a label we apply and that its persistence conditions are determined by our linguistic conventions and practical interests, not by a deep metaphysical fact. This view suggests that the paradox arises from:

a failure to recognize the true, essential properties that constitute the ship's identity.
an incorrect application of psychological continuity criteria to an inanimate object.
the ambiguity in our criteria for tracking objects over time, which are practical rather than absolute. (correct answer)
a misunderstanding of the physical laws governing the conservation of matter and energy.

Explanation: This philosophical position is a form of conventionalism or anti-realism about identity over time. It suggests that there is no single 'correct' answer to the paradox because our concepts for tracking identity (like 'ship') are not precise enough to handle such edge cases. The 'correct' answer depends on what we value or find useful (e.g., historical trace, material composition, function). Thus, the paradox stems from the ambiguity in our tracking criteria. A argues for the opposite view (essentialism). B is incorrect because the argument isn't about misapplying a specific theory, but about the nature of all such theories. D is irrelevant as the problem is metaphysical, not physical.

Question 5

A person, Sam, is teleported to Mars. The machine scans and destroys Sam's body on Earth. However, the transmission is intercepted and used to create two perfect replicas: Sam-A on Mars and Sam-B on Venus. Sam-A and Sam-B have identical memories and psychological states up to the moment of teleportation.

The creation of two replicas from one original primarily demonstrates that psychological continuity (Relation R, in Parfit's terms) lacks a logical property required for identity. What is that property?

Reflexivity (everything is identical to itself).
Symmetry (if A is identical to B, then B is identical to A).
Intrinsicness (identity depends only on the object itself).
Transitivity (if A is identical to B, and B is identical to C, then A is identical to C). (correct answer)

Explanation: The issue is a violation of the logic of identity, which is an equivalence relation (reflexive, symmetric, and transitive). In this fission case, Sam is psychologically continuous with Sam-A, and Sam is psychologically continuous with Sam-B. If psychological continuity were identity, this would mean Sam = Sam-A and Sam = Sam-B. By transitivity, it would follow that Sam-A = Sam-B. But clearly, Sam-A and Sam-B are two different people in two different places. Since the conclusion is false, the premise (that psychological continuity is identical to identity) must be flawed. The relationship is not transitive and cannot be identity. A and B are not the properties being tested here. C is a philosophical feature, not a logical one, though fission also shows that 'survival' can depend on extrinsic facts (whether other copies exist).

Question 6

A person, Alex, uses a teleportation device. The device scans Alex's body, destroying it in the process, and transmits the information to Mars, where an exact physical replica is created from new matter. The replica, Alex-2, has all of Alex's memories, personality traits, and consciousness.

According to Derek Parfit's psychological criterion for personal identity, which of the following statements presents the most significant challenge to the claim that Alex and Alex-2 are the same person?

The physical body of Alex-2 is composed of entirely new matter, breaking the chain of physical continuity.
The teleportation process involves the destruction of the original person, Alex, which makes any subsequent continuity impossible.
If the teleporter malfunctioned and created two replicas (Alex-2 and Alex-3), Alex could not be identical to both without violating the transitivity of identity. (correct answer)
An immaterial soul might not be transferred during the scanning process, meaning the core of Alex's identity is lost.

Explanation: Parfit's psychological criterion focuses on overlapping chains of strong psychological connectedness (Relation R). The standard teleportation case seems to satisfy this. However, the most profound problem, which Parfit himself explores, is the 'Fission' or 'Branching' problem. If Alex can be psychologically continuous with both Alex-2 and Alex-3, but Alex-2 and Alex-3 are not the same person as each other, then identity (which is a one-to-one relation) cannot be what matters. This forces a distinction between identity and survival. A is a challenge for a physical/bodily criterion, not a psychological one. B is the event that prompts the question, not the challenge to the psychological criterion itself. D introduces the soul theory, which is a different criterion for identity altogether and not the focus of Parfit's psychological view.

Question 7

If a person agrees to be teleported, knowing that the process involves destruction and re-creation, their willingness might suggest they believe personal identity is preserved. This belief is most consistent with a theory of identity that prioritizes:

the persistence of a unique, immaterial soul that is transferred.
the continuity of the physical body, assuming the replica is a continuation.
the preservation of a pattern, such as psychological states and functional organization. (correct answer)
the mereological fact that the same number of atoms is used in the replica.

Explanation: Willingly undergoing teleportation implies that what matters for one's survival is not the persistence of a specific chunk of matter (the body) or an unobservable soul, but the continuation of the 'self' as defined by memories, personality, and conscious experience. This is essentially a belief that identity consists in the preservation of a pattern or structure. This aligns perfectly with functionalist and psychological continuity theories. A is a possible belief, but it requires an extra assumption about how a soul would be 'transferred' by such a physical process. B is inconsistent, as bodily continuity is explicitly broken. D is irrelevant, as the identity of atoms does not matter, only that they are arranged in the same way.

Question 8

A historian is trying to decide which of two ships is the 'real' Ship of Theseus: Ship B (the gradually repaired one) or Ship C (the one reassembled from old planks). The historian decides that Ship C is the real one, arguing that its material provides a direct physical link to the historical events the ship was part of, which is what matters for historical artifacts. This reasoning treats the ship's identity as being most analogous to:

a living organism's identity, which persists through metabolic replacement of cells.
a person's identity under a psychological criterion, where memory provides a link to the past.
a person's identity under a bodily criterion, where the persistence of the physical form is key. (correct answer)
a brand's identity, which can be applied to many different products made in different factories.

Explanation: The historian's focus on the 'direct physical link' provided by the original material is analogous to a bodily or physical criterion of personal identity. Just as a body theorist would say that personal identity is tied to the persistence of a specific physical human animal, this historian is arguing that the artifact's identity is tied to the persistence of its specific physical matter. A is a poor analogy because organisms do replace their matter, which is the position the historian is arguing against. B is also a poor analogy because the historian is focused on physical material, not an analogue of memory or psychological states. D is the opposite of the historian's reasoning; a brand has an abstract identity separate from any particular physical instance.

Question 9

In a variation of the Ship of Theseus thought experiment, the original ship (Ship A) has its planks replaced one by one, resulting in a completely new ship (Ship B) in the same location. The old planks are reassembled into a ship (Ship C) in an adjacent dock. A philosopher who subscribes to a spatiotemporal continuity theory of identity would most likely be forced to conclude that:

Ship A and Ship B are the same ship, because Ship B occupies the continuous spatio-temporal path originating from Ship A. (correct answer)
Ship A and Ship C are the same ship, because Ship C is composed of the original material of Ship A.
Neither Ship B nor Ship C can be Ship A, as the branching of identity into two candidates makes the original identity cease to exist.
Ship A's identity is transferred to whichever ship, B or C, continues to perform the primary function of a ship.

Explanation: The spatiotemporal continuity theory posits that identity is preserved through a continuous path in space and time. Ship B is the object that maintains this continuity, as it is the result of a gradual transformation in the original location. Ship C, despite being made of original parts, appears in a new location and does not share this continuous history. Therefore, a strict adherent to this view is led to the conclusion that A and B are the same ship. Distractor B describes a mereological (parts-based) theory. Distractor C points to the problem of fission, which some philosophers (like Parfit) argue leads to the cessation of identity, but it is not the primary conclusion of the spatiotemporal continuity theory itself. Distractor D describes a functionalist theory of identity.

Question 10

A person undergoes a procedure where their neurons are gradually replaced, one by one, with synthetic nano-machines that perfectly replicate the function of the original neurons. The process takes ten years, during which the person's consciousness, memories, and personality remain completely uninterrupted. At the end of the process, no original biological matter remains in their brain.

This thought experiment primarily challenges the coherence of which view of personal identity?

Psychological continuity theory, because the person's mental states are artificially maintained.
Functionalism, because the function of the brain is preserved by different underlying components.
Substance dualism, because it demonstrates the dependence of mind on a physical substrate.
The bodily criterion (or animalism), because identity is maintained despite a complete change in physical substance. (correct answer)

Explanation: This scenario is a version of the Ship of Theseus applied to a person. The core challenge is directed at theories that ground personal identity in the persistence of the physical body or organism. Here, the organism's material composition is completely replaced, yet intuition suggests the person remains the same due to uninterrupted psychological and functional continuity. This directly contradicts the bodily criterion, which would imply the original person ceases to exist. A is incorrect because the scenario is designed to satisfy psychological continuity perfectly. B is incorrect because functionalism is supported, not challenged, by this example. C is challenged, but the primary target related to identity persistence is the bodily criterion.

Question 11

A proponent of a 'closest continuer' theory of identity is faced with a teleportation case where the original person is scanned and destroyed, and two replicas are created, Replica A and Replica B. Replica A is a 99.9% perfect copy, while Replica B is a 99.8% perfect copy. How would a 'closest continuer' theorist most likely resolve the identity of the original person?

The original person is identical to Replica A, as it is the most psychologically and physically similar successor. (correct answer)
The original person is identical to both Replica A and Replica B, as both are sufficiently similar continuers.
The original person does not survive, because the existence of more than one potential continuer invalidates any claim to identity.
The original person's identity is indeterminate until one of the replicas is destroyed, leaving a single unique continuer.

Explanation: The 'closest continuer' theory, proposed by Robert Nozick, states that something is identical to a later thing if the later thing is 'close enough' to the original and is also the 'closest' continuer of the original. In this case, Replica A is closer than Replica B. As long as Replica A meets the 'close enough' threshold, it would be identified as the original person, because it has no competitor of equal or greater closeness. B violates the principle that identity is a one-to-one relation. C describes a different solution to the fission problem (like Parfit's), which holds that identity ceases in branching cases, even if a 'closest' continuer exists. D introduces an unsupported condition about future events determining present identity.

Question 12

Consider the Ship of Theseus scenario where the replaced ship is called Ship B and the reassembled ship is Ship C. A philosopher claims that Ship B is the Ship of Theseus. Another philosopher claims Ship C is the Ship of Theseus. A third philosopher argues that both claims are mistaken because identity requires 'non-branching' causal chains. The third philosopher's argument implies that:

the original Ship of Theseus ceases to exist the moment the first plank is replaced.
the original Ship of Theseus ceases to exist at the point where a second viable candidate for its identity is created. (correct answer)
one of the ships must be destroyed for the other to rightfully inherit the identity of the Ship of Theseus.
the ship's identity was never tied to its physical form but to its name and historical record.

Explanation: The 'non-branching' constraint is crucial for theories of identity, especially psychological continuity theories applied to persons, but the logic extends here. The idea is that identity can only be passed to a unique successor. The Ship of Theseus problem creates two plausible successors (B and C). A philosopher who insists on a non-branching chain would argue that as soon as the second candidate (Ship C) comes into existence and has a plausible claim to the identity, the original's identity can no longer be uniquely traced. At that point, the original ceases to exist, and we have two new ships. A is too strong; replacing one plank doesn't create a branching problem. C suggests a way to resolve the branching, but the argument is about what happens when branching occurs. D describes a different theory of identity altogether (a conventionalist or narrative one).

Question 13

In a teleportation variant, a scanner creates a perfect replica of a person, but due to a transmission lag, the original person is not destroyed until one full second after the replica has been activated on Mars. For that one second, two identical persons exist.

This one-second overlap most severely undermines which of the following claims?

The claim that the replica is qualitatively identical to the original.
The claim that the replica is numerically identical to the original. (correct answer)
The claim that psychological continuity is a necessary condition for personal identity.
The claim that bodily continuity is a sufficient condition for personal identity.

Explanation: Numerical identity means being one and the same thing. It is a fundamental logical principle that one thing cannot be in two different places at the same time. The one-second overlap, where two distinct bodies exist simultaneously, makes it logically impossible to claim they are numerically identical. They are two things, not one. A is incorrect because the scenario stipulates they are perfect replicas, so they are qualitatively identical. C is not undermined; in fact, one could argue psychological continuity has branched, which is a problem for it, but the most direct logical impossibility relates to numerical identity. D is not undermined; in fact, the original person's persistence for that one second supports the bodily continuity view for that individual.

Question 14

A philosopher analyzing the Ship of Theseus argues for 'four-dimensionalism' (or perdurantism), the view that objects are four-dimensional 'spacetime worms' that have temporal parts at different times. How does this view propose to solve the paradox?

By arguing that only one of the resulting ships can be a genuine four-dimensional object, while the other is an illusion.
By denying that objects can persist through time, thus dissolving the problem of identity entirely.
By stating that the repaired ship and the reassembled ship are distinct objects that share some temporal parts with the original ship. (correct answer)
By claiming that the ship's identity is defined by its final temporal part, making the repaired ship the true successor.

Explanation: Four-dimensionalism resolves the paradox by treating objects as extended in time just as they are in space. From this perspective, there is an original spacetime worm (the original ship). When the parts are replaced and reassembled, two new spacetime worms come into existence. The repaired ship is a spacetime worm that shares spatial location and some causal continuity with the original. The reassembled ship is a different spacetime worm that shares material parts with the original. The paradox is solved by saying they are simply two different objects that have a specific relationship (sharing temporal parts) with the original object. There is no single 'true' successor, just different overlapping entities. B is incorrect; 4D-ism is a theory of persistence, not a denial of it. A and D propose arbitrary solutions not inherent to the four-dimensionalist framework.

Question 15

In the classic Ship of Theseus problem, if one were to prioritize the ship's historical continuity and its unbroken chain of ownership and function as a vessel named 'Theseus', which criterion of identity is being implicitly favored?

Mereological identity (based on parts).
Spatiotemporal continuity.
A narrative or relational theory of identity. (correct answer)
Qualitative identity (based on properties).

Explanation: Prioritizing factors like name, ownership, and history points toward a narrative or relational view. This view holds that identity is constituted by a story or a set of social and historical relations. The ship that continues to be called 'Theseus', that is registered as 'Theseus', and that people point to as 'Theseus' would be the true ship, regardless of its material or spatiotemporal path. A is incorrect because this view ignores the parts. B is related, as the repaired ship has spatiotemporal continuity, but the emphasis on name and history is better captured by a narrative theory. D is incorrect because the repaired ship is not qualitatively identical to the original at any given time.

Question 16

A key objection to accepting a simple psychological continuity theory of identity is that it seems to treat a person as a 'series' of mental states rather than a 'substance' that has them. The teleportation thought experiment highlights this objection by suggesting that:

a series of mental states can be copied, while a unique substance cannot, making the replica a mere duplicate. (correct answer)
mental states cannot be accurately scanned or transmitted, making perfect psychological continuity impossible in principle.
a person's substance is their body, and since the body is destroyed, identity is necessarily lost.
the continuity of consciousness is an illusion, and we are all new persons from moment to moment.

Explanation: This question gets at a classic 'substance vs. bundle' debate. If a person is a unique substance (like a soul or a particular body), then copying their properties (their mental states) doesn't copy them. You just create a new substance with the same properties. The teleportation experiment makes this vivid: it suggests you can copy the 'pattern' or 'series' of psychological states, but this doesn't guarantee you've transferred the original subject. This supports the intuition that the replica is a new person who just thinks they are the original. B is a practical objection, not a philosophical one. C states the conclusion of the bodily theory but doesn't explain why the psychological theory is problematic in this context. D is a more radical Humean view that dissolves the problem rather than addressing the specific objection.

Question 17

Mereological identity (based on parts).
Spatiotemporal continuity.
A narrative or relational theory of identity. (correct answer)
Qualitative identity (based on properties).

Question 18

Imagine a modified teleportation scenario. A scanner reads a person's information without destroying the original. It creates a perfect replica on a distant planet. The original person watches the replica appear on a screen and then continues with their life. Which concept is most directly tested by this 'non-destructive duplication' scenario?

The distinction between numerical identity and qualitative identity. (correct answer)
The role of gradual change versus sudden change in preserving identity.
The functionalist requirement that an object must perform its original role to maintain identity.
The mereological principle that identity is solely constituted by the sum of an object's parts.

Explanation: This scenario creates two beings who are, by definition, perfectly similar in every way (qualitatively identical) but are clearly not the same single entity (they are not numerically identical). The original person exists simultaneously with the replica, making it impossible to claim they are one and the same person without contradiction. This forces a clear distinction between being 'exactly like' someone and 'being' that someone. B is more relevant to the Ship of Theseus. C and D are theories of identity, but the core issue highlighted here is the nature of identity itself as a unique relation, which is captured by the numerical/qualitative distinction.

Question 19

Ship A and Ship B are the same ship, because Ship B occupies the continuous spatio-temporal path originating from Ship A. (correct answer)
Ship A and Ship C are the same ship, because Ship C is composed of the original material of Ship A.
Neither Ship B nor Ship C can be Ship A, as the branching of identity into two candidates makes the original identity cease to exist.
Ship A's identity is transferred to whichever ship, B or C, continues to perform the primary function of a ship.

Question 20

The physical body of Alex-2 is composed of entirely new matter, breaking the chain of physical continuity.
The teleportation process involves the destruction of the original person, Alex, which makes any subsequent continuity impossible.
If the teleporter malfunctioned and created two replicas (Alex-2 and Alex-3), Alex could not be identical to both without violating the transitivity of identity. (correct answer)
An immaterial soul might not be transferred during the scanning process, meaning the core of Alex's identity is lost.