2.3. Category Mistakes and Level Confusion: The Ship of Theseus as an Example
In addressing the enduring puzzle of the Ship of Theseus, a highly influential and intuitively attractive solution has been proposed, represented by David Wiggins (1980). This solution asserts that the identity of an object is not guaranteed by its properties at any single moment, but must be borne by its continuity in spacetime and an uninterrupted historical causal path. The great advantage of this approach is that it successfully captures our deep intuition about “objects persisting through time”—that things are not instantaneous existences but have their life histories (careers) or biographies.
Another solution, perdurantism (four-dimensionalism), provides a radically different and metaphysically elegant picture (see Heller, 1984; Sider, 2001). With its thorough clarity, this theory ingeniously avoids many traps of diachronic identity. Perdurantism asserts that the Ship of Theseus is not a three-dimensional entity that “fully exists” through time, but a “spacetime worm” extended in four-dimensional spacetime. Each time slice of the ship is regarded as a temporal part of this four-dimensional object. Thus, so-called “change” merely means that this four-dimensional whole has different properties (such as different planks) at different temporal parts. In this framework, the original Ship of Theseus (as a four-dimensional entity), the replaced ship, and the ship reassembled from the old planks are three different four-dimensional objects. They may have completely identical three-dimensional cross-sections at some time segment (thus indistinguishable at that moment), but as wholes, they are naturally different. The excellence of this solution lies in transforming the problem of persistence from the troubling “identity” to the relatively clear “part-whole” relation.
Closely related to perdurantism is stage theory (see Sider, 1996; Hawley, 2001), which, while retaining many advantages of perdurantism, attempts to better accommodate our everyday language intuition that “objects are three-dimensional.” Stage theorists argue that what we usually call “the Ship of Theseus” refers not to the entire four-dimensional worm, but to one of its stages or time slices at a specific time point. When we say at time t that “the ship is the same,” we are actually saying that there exists a (primitive) counterpart relation between the stage at t and the stage at an earlier time t1, maintained by some similarity and causal continuity. Stage theory, with its conceptual economy, avoids presupposing identity relations across time, thus exhibiting strong theoretical appeal.
However, although the above theories are each ingeniously crafted in their internal logic and often self-consistent, this paper will argue that they face a common, profound dilemma at the normative level. Whether appealing to historical paths, four-dimensional wholes, or counterpart relations, these theories all attempt to provide a single, absolute criterion for identity judgment. To achieve this goal, they must construct “time” or “spacetime position” itself as a constitutive element of the object’s identity. This means that within their theoretical frameworks, answering the question “Is the ship at t1 the same as at t2?” logically necessarily depends on checking spacetime coordinates or cross-time associations.
This paper argues that it is precisely this key theoretical move that inadvertently leads to a “category mistake.” Let us formally reconstruct the solutions of each theory: they actually adopt a domain defined as (ship’s physical properties, historical causal path/four-dimensional whole/counterpart relation).
Let us apply n ≡ n to the Ship of Theseus. First, we must clarify the question: when we ask “Is the replaced ship still the original ship?”, what is the default comparison category? A reasonable interpretation is that we care about its identity as an “objectively verifiable ship,” that is, the category (physical ship), with relevant properties {e.g., all particles of the ship and their arrangement shape}.
Now, let us examine the historical path theory. This theory, in answering the above question, actually introduces a new category (physical ship, history), with relevant properties {all particles of the ship and their arrangement shape, historical causal path}. In the (physical ship, history) category, since the historical path has changed, the replaced ship is naturally different from the original ship.
This paper argues that the controversy here stems from confusion of categories. The questioner implicitly asks within the (physical ship) category, while the historical path theory answers within the (physical ship, history) category. These two answers—”yes” {in (physical ship)} and “no” {in (physical ship, history)}—do not contradict because they answer two different questions. Imposing the answer from (physical ship, history) onto the question from (physical ship) constitutes a category mistake. The true value of historical path theory lies in revealing “history” as an important category, but it erroneously treats it as the sole decisive category.
We can formally reconstruct this as follows:
1. Initial question: Determine whether the entity “ship” at time points t1 and t2 is the same.
2. Correct (n) domain (based on the initial question): Should include only properties related to the “ship” substance, i.e., physical ship: (all particles of the ship and their arrangement shape)
3. Category mistake in historical/four-dimensional/stage theories:
· Wiggins actually adopts history + physical ship (all particles of the ship and their arrangement shape, historical causal path).
· Perdurantism actually adopts 4D + physical ship (all particles of the ship and their arrangement shape, spacetime coordinates).
· Stage theory actually adopts stage + physical ship (all particles of the ship and their arrangement shape, counterpart relation).
The process leading to this can be formally expressed as:
· They attempt to answer the identity question based on (physical ship): “Is Ship at t1 ≡ Ship at t2?”
· However, the judgment domain they actually use is (physical ship + history, 4D, or stage).
· Since the materials in the replacement process are identical to the original materials, we have (physical ship_t1) = (physical ship_t2).
· But because the historical path, spacetime coordinates, or counterpart relation has changed, (physical ship_t1) ≠ (physical ship + history/spacetime/stage_t2).
· Thus, they conclude Ship_t1 ≠ Ship_t2 to answer the physical ship question.
Thus, we see an interesting situation: each theory effectively answers a question, but possibly not the one initially posed. They precisely answer “Is there a continuous four-dimensional worm connecting the ship at t1 and t2?” or “Is the stage at t2 the counterpart of the stage at t1?”, but treat this answer as the ultimate adjudication of the question “Is the ship structurally the same?” This is like a judge, asked “Does the defendant’s behavior comply with Criminal Law Article X?”, giving a verdict after consulting the civil law code. The conclusion may be coherent within its own system, but it has quietly switched the venue of the debate, which is a category mistake.
Therefore, the true contribution of historical path theory may lie in its excellence in revealing how various explanatory properties such as “history” and “spacetime whole” influence our identity judgments. But its limitation is that it attempts to elevate such explanatory properties to metaphysical necessities, thus having to expand the criteria for identity questions to maintain the integrity of its theory. The value of this theory is that it does not need to make this difficult expansion, but by clarifying the levels of the question, allows different domains/properties to give effective and non-conflicting answers to different questions. It does not solve these puzzles but dissolves them.
2.4. Conservation
2.4.1. The Indistinguishability of Identical Particles in Quantum Mechanics Poses the Most Severe Challenge to Leibniz’s PII (Citations 1,2), Yet Provides a Natural, Physically Evidenced Model for This Theory
Current philosophical discussions on identical particles, facing the challenge of quantum identical particles to PII, mainly fall into two categories of mainstream solutions: revisionism and revolutionism. The former attempts to salvage some form of individuality principle, while the latter abandons individuality itself.
Saunders’s solution undoubtedly represents one of the most ingenious and technically rigorous attempts in the revisionist path. By ingeniously defining “weak discernibility,” he successfully liberates the discussion from the dead end of intrinsic properties, providing an insightful perspective for finding the cornerstone of individuation in relations. The complexity of this approach and the extensive discussions it has sparked in itself prove its profound philosophical value. However, it is precisely this technical complexity that exposes a potential cost underlying the solution: its definition of “purely extensional relational properties,” though striving for precision, inevitably introduces considerable terminological ambiguity, to the extent that its defenders must also carefully handle accusations of circular argumentation (Muller & Saunders, 2008). More centrally, the entire theoretical edifice of this solution is built on an unsettling presupposition: that the “individuality” of identical particles must, and can only, be “saved” by finding some (even relational) individuating property. This makes its theoretical efforts—no matter how ingenious—essentially an ad-hoc repair to salvage a premise. When applied to states with indefinite particle number in quantum field theory, this strategy of continuously introducing new relational properties to salvage individuality becomes increasingly ad-hoc: it no longer resembles an elegant deduction of the theory itself, but more like an increasingly costly price paid to maintain the theoretical premise (that individuality must exist).
Facing the dilemmas of revisionism, another revolutionary solution chooses a more thorough path. Scholars represented by Décio Krause (2011) propose a highly subversive argument: quantum particles may not be “individuals” in the traditional metaphysical sense at all. Therefore, laws based on individual identity are categorically wrong from the start. They should be understood as “non-individuals” and described using highly specialized mathematical tools such as quasi-set theory.
Krause’s solution is striking for its conceptual thoroughness and consistency; it unreservedly embraces the most counterintuitive features of quantum mechanics, decisively breaking with our entire classical framework of objects and spacetime positioning. This resolute posture is undoubtedly clean and efficient in theory. However, the corresponding cost of this efficiency is that the concept of “non-individual” itself imposes considerable explanatory burden in metaphysics, requiring us to abandon an entire set of deeply rooted intuitive understandings of what “one thing” means.
2.4.2. This Paper’s Solution: A Hierarchical Relativity Framework
The above two solutions share a deep misconception: they both attempt to find answers to a wrongly posed question. The problem is not “What is the correct individuating property?”, but “At what category are we inquiring about the identity question?”
This paper provides a meta-framework for this. We define an observable particle state as: P = (o, q), where o is the set of intrinsic properties (mass, charge, spin, etc.), and q is the set of spacetime coordinates.
· When we inquire at the level of (particle) = o, i.e., comparing only intrinsic properties, all identical electrons are (electron) = (mass m_e, charge -e, spin 1/2...). According to Axiom 1, at this level, they are indeed the same electron e. This explains the root of their indistinguishability.
· When we inquire at the level of (particle state) = (o, q), since q (such as position) is necessarily different, (P1) ≠ (P2), so they are different particle states. This explains why we observe multiple scattering events in experiments.
Thus, the confusion brought by quantum identical particles stems from erroneously invading the difference at the q (spacetime coordinates) level into the identity judgment at the o (intrinsic properties) level. Steins Theory resolves the contradiction by clearly distinguishing these two levels: they are both “one” (as a logical concept) and “many” (as manifestations in specific spacetime). Particle annihilation and creation merely represent the decoupling and re-coupling of e with different coordinates q.
The advantage of this solution is that it absorbs the advantages of Krause’s solution in acknowledging the specificity of quantum mechanics (by interpreting “non-individuality” as identity at the o level), while avoiding its radical metaphysical cost (we are still talking about “quanta,” just in different categories); at the same time, it explains why Saunders’s strategy of introducing relational properties seems feasible in some cases (because he erroneously took q-level properties as the basis for individuation at the o level), yet fundamentally misguided.
2.4.3. Formal Derivation Proof of Conservation:
Let the basic particle state be expressed as: Particle P = (o, q) where:
· o is the set of intrinsic properties (such as mass m, charge q, spin s)
· q is the set of spacetime coordinates (such as position x, time t).
Formalization:
1. When the domain of p is P = (o, q1), a certain coordinated electron
2. Coordinate decoupling (destruction): (o, q1) → (o), (q1) ⇒ The particle degenerates to a pure eigenstate (o), unmeasurable due to lack of observable basis (q = ∅). ∀ particle states (o, q1) and (o, q2), it can be found that: (o) ≡ (o) indicates:
· When the eigenproperties of two particles are indistinguishable (o ≡ o), regardless of how their spacetime coordinates q1 ≠ q2 differ, their particle is the same electron e = (o) projected in different spacetimes
Physical interpretation:
· Particle annihilation ⇨ Set decoupling rather than extinction ⇒ e = (o) enters a free state
· Particle creation ⇨ The same e binds new coordinate q2 ⇒ Observed as reappearance, example: Electron e disappears at position q1 and appears at q2, actually the coordinate migration of electron e = (q=-1e, m_e, s=1/2...): (e, q1) → (e) → (e, q2), its electron identity guaranteed by n ≡ n.
Direct corollary: Conservation theorem: What logic allows exists, will not annihilate nor update
2.5. Symmetry
Max Black’s (1952) symmetric universe thought experiment poses the most extreme challenge to Leibniz’s strong PII. He imagines a universe with only two completely identical spheres. These two spheres are indistinguishable in all intrinsic properties (mass, composition, shape, etc.) and all relational properties (distance X miles apart, symmetric to each other). Black thereby argues that this is a genuine scenario of “two” things, thus refuting PII—there is no property to distinguish them, yet they are still numerically two distinct entities.
Traditional response strategies mainly fall into two types: one questions the metaphysical possibility of such a symmetric universe (e.g., requiring a basis for “numerical difference” itself, which usually loops back to some hidden property); the other, like Saunders (2003), argues that relational properties (such as “being X miles from a sphere”) can themselves serve as weakened distinction bases. However, the former is criticized as ad-hoc, while the latter is difficult to work in Black’s original setting because each sphere’s relational properties (“being X miles from another sphere”) remain completely identical.
This paper argues that Black’s challenge and the dilemmas of traditional responses jointly root in an unexamined presupposition: that “numerical two” is a primitive, irreducible fact. This theory provides a brand-new analytical perspective. Under this paper’s framework, we must first clarify the domain of (n).
· If (sphere) is defined as the set of all traditional properties (intrinsic + relational), i.e., (n) = {mass M, shape spherical, ..., distance X from a sphere}, then according to the axiom, since (n) ≡ (n), we inevitably conclude sphere ≡ sphere. This seems to directly derive the PII conclusion that Black attempted to refute.
· However, Black’s intuition—”there are clearly two spheres here”—is not entirely baseless. This theory interprets it as a fixed pattern in thinking. The reason observers report “seeing two” is because their perspective itself is embedded in this symmetric spacetime coordinate system. This paper argues that a fundamental misconception shared by Black and his commentators is assuming that the reference of “sphere” and “sphere” necessarily corresponds to two entities with independent spacetime coordinates. This presupposition forces them into a dilemma between “abandoning PII” or “inventing new metaphysical concepts.” The concept of “Coordinate Self-Reference” provides a third way out of this dilemma. Formalization: For the entire symmetric system S, define: (S) = {there exists a sphere with property set P, and the sphere is opposite itself}. This description looks complex, but simply put, (S) describes a single coordinate framework that allows “self-facing.” Within this framework, a sphere being opposite itself is not a grammatical error, but an accurate description of a singularity coordinate topology. Visually, the “two” spheres presented are projections of this single, self-referential coordinate structure in Euclidean space perception (similar to an object and its mirror image, but here there is no mirror; it is the topological property of space itself).
· System S: (S) describes the state after a single sphere is bound to a special self-referential coordinate topological structure: (sphere, R_self-facing).
· Paradox dissolution: Black’s error lies in erroneously inferring the existence of two spheres (sphere_1, sphere_2) from the system state (sphere, R_self-facing). He confused categories, using the description result of (S) to answer the question about (single sphere). In fact, there never existed a second sphere; what always existed was only one sphere, in a special coordinate topology that produces a “double image projection.”
Thus, this framework does not deny our intuition of “seeing two spheres,” but provides a brand-new, more precise ontological explanation for this intuition: it is the perception of a single sphere in a self-referential coordinate topology. This successfully resolves the apparent contradiction between PII and counting intuition, while avoiding the introduction of any ad-hoc individuating factors. Black’s challenge not only fails to refute the law of identity but, through the level analysis of this framework, more profoundly reveals the dependence of “identity” judgments on the background framework.