2. Analysis
2.1. Axioms
2.1.1. Axiom 1 (Self-Identity)
∀n ∈ I, n ≡ n. (n) is necessarily identical with itself and forms the foundation of logical reference.
2.1.2. Axiom 2 (Distinctness)
∀n ∀m (n ≠ m → ∃F (F(n) ∧ ¬F(m)))
Core corollary: Two (n) with identical content must be the same (see section 2.5).
Note: See examples in 3.1, Chapter 2, and 3.7. Unless explicitly stated otherwise (for example, the question in 3.7 specifies consideration from a cognitive perspective), this paper defaults to the premise that propositions are effective in reality (physics). This default is because if a proposition does not pursue real-world effectiveness, any unexpected errors in its conclusions due to unstated shifts in perspective are naturally unimportant.
2.2. Category Mistakes and Level Confusions: The Case of the Ship of Theseus
When addressing the enduring puzzle of the Ship of Theseus, one highly influential and intuitively appealing solution has been proposed, represented by figures such as David Wiggins (1980). This approach holds that an object’s identity is not guaranteed by its instantaneous properties at any given moment but must be carried by its spatiotemporal continuity and an uninterrupted historical causal path. The great advantage of this approach lies in its successful capture of our deep intuitions about “objects persisting through time”—that things are not instantaneous existences but possess a life history or biography.
Another approach, four-dimensionalism (Perdurantism), offers a starkly different and metaphysically parsimonious picture (see Heller, 1984; Sider, 2001). This theory, with its thorough clarity, very cleverly avoids many pitfalls of diachronic identity. Four-dimensionalism holds that the Ship of Theseus is not a three-dimensional entity that “exists wholly” in 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 possesses different properties (such as different planks) in different temporal parts. Under this framework, the original Ship of Theseus (as a four-dimensional entity), the ship after replacement, and the ship reassembled from old planks are three different four-dimensional objects. They may possess completely identical three-dimensional cross-sections at certain time segments (and thus be indistinguishable at that moment), but as wholes, they are naturally distinct. The excellence of this approach lies in transforming the problem of persistence from the troubling question of “identity” into the relatively clear relation of “part-whole.”
Closely related to four-dimensionalism is Stage Theory (see Sider, 1996; Hawley, 2001), which retains many advantages of four-dimensionalism while attempting to better accommodate our everyday linguistic intuition that “objects are three-dimensional.” Stage theorists hold that what we ordinarily call the “Ship of Theseus” refers not to the entire four-dimensional worm but to a stage or time-slice of it 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 earlier time t₁, maintained by some similarity and causal continuity. With its conceptual economy, Stage Theory avoids presupposing a cross-temporal identity relation and thus exhibits strong theoretical appeal.
However, despite the ingenuity and often self-consistency of the internal logic of the above theories, 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. To achieve this goal, they must construct “time” or “spatiotemporal location” itself as a constitutive element of an object’s identity. This means that, within their theoretical framework, answering the question “Is the ship at t₁ and t₂ the same?” logically necessarily depends on checking spatiotemporal coordinates or cross-temporal associations.
This paper argues that it is precisely this key theoretical move that inadvertently leads to a “category mistake.” Let us formally reconstruct the schemes of each theory: they actually adopt a domain of: (physical properties of the ship, 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 category of comparison? A reasonable interpretation is that we are concerned with its identity as an “objectively verifiable ship,” i.e., the category (physical ship), with relevant attributes {e.g., all particles of the ship and their arrangement and shape}.
Now, let us examine the historical path theory. When answering the above question, this theory actually introduces a new category (physical ship, history), with relevant attributes {all particles of the ship and their arrangement and shape, historical causal path}. Under 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 arises from a confusion of categories. The questioner is implicitly asking within the (physical ship) category, while the historical path theory answers within the (physical ship, history) category. The two answers—”yes” {under (physical ship)} and “no” {under (physical ship, history)}—are not contradictory, because they answer two different questions. Imposing the answer from (physical ship, history) onto the question of (physical ship) constitutes a category mistake. The true value of the historical path theory lies in revealing “history” as an important category, but it erroneously treats it as the sole decisive category.
We can reconstruct this formally as follows:
1. Initial question: Determine whether the entity “ship” at time points t₁ and t₂ is the same.
2. Correct (n) domain (according to the initial question): Should contain only attributes related to the “ship’s” matter, i.e., physical ship: (all particles of the ship and their arrangement and shape).
3. Category mistake of historical/four-dimensional/stage theories:
· Wiggins effectively adopts history + physical ship (all particles of the ship and their arrangement and shape, historical causal path).
· Four-dimensionalism effectively adopts 4D + physical ship (all particles of the ship and their arrangement and shape, spatiotemporal coordinates).
· Stage Theory effectively adopts stage + physical ship (all particles of the ship and their arrangement and 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 exactly the same as the original, we have (physical ship_t1) = (physical ship_t2).
· But because the historical path, spatiotemporal coordinates, or counterpart relation has changed, (physical ship_t1) ≠ (physical ship + history/spacetime/stage_t2).
· Thus, they reach the conclusion Ship_t1 ≠ Ship_t2 to answer the physical ship question.
We thus see an interesting situation: each theory effectively answers a question, but perhaps not the one originally posed. They precisely answer “Does there exist a continuous four-dimensional worm connecting t₁ and t₂ for the ship?” or “Is the stage at t₂ a counterpart of the stage at t₁?”, but then treat this answer as the ultimate verdict on “Is the ship structurally the same?” This is like a judge being asked “Does the defendant’s behavior comply with Article X of the Criminal Law?” but consulting the Civil Code and issuing a verdict. The conclusion may be coherent within its own system, but it has quietly shifted the arena of debate—this is a category mistake.
Therefore, the true contribution of the historical path theory may lie in its excellent revelation of how “history,” “spatiotemporal whole,” and other attributes, as powerful explanatory properties, influence our identity judgments. Its limitation lies in attempting to elevate such explanatory properties to metaphysical necessities, thereby having to expand the criteria for judging identity questions to maintain theoretical completeness. The value of this theory lies in the fact that it does not need to make such difficult expansions but, by clarifying the attribute sets of the questions, allows different domains/attributes to provide effective, non-conflicting answers to different questions. Not solving these difficulties, but dissolving them.
2.3. Relation to Relative Identity?
When discussing the solution proposed in this paper, one unavoidable theory is Peter Geach’s “relative identity” theory. Frankly speaking, Geach’s work has a surprisingly close affinity with this paper in intuition: both reject the naive notion of “absolute identity” and hold that “whether x and y are identical” cannot be discussed in a vacuum detached from some standard or category. Geach acutely observed that in ordinary language and philosophical discussion, we never merely say “x and y are identical” but always say “x and y are the same ship,” “x and y are the same person,” or “x and y are the same word-type.” His core thesis—that identity is relative and depends on the sortal noun A in “the same A”—undoubtedly captures the essence of a great deal of real linguistic practice. Moreover, Geach insightfully emphasized that things within the same domain of objects can appear identical or non-identical under different standards of identity (i.e., different “same A” predicates), without the things themselves changing. This insistence on “the things themselves not changing” makes his theory particularly elegant and economical in metaphysics.
On the surface, the theory in this paper is extremely similar to Geach’s relative identity. Taking the Ship of Theseus as an example: Geach would say that under the standard of “same ship” (as a material object), the old ship and the new ship are the same; under the standard of “same ship-with-history” (with a historical trajectory), they are not. This is outwardly identical to this paper’s distinction in section 2.2 between the two categories of (physical ship) and (physical ship, history). For this reason, a careful reader might well think that this paper is merely restating an already existing insight with new terminology.
However, this paper attempts to show that this surface similarity masks a deeper divergence in presuppositions that Geach himself and his followers failed to examine. To see this clearly, we need a more detailed reconstruction of Geach’s thesis than in the original.
Let us turn to page ten of the original text on relative identity: “The words in my book (say) will constitute just one universe of discourse; but different criteria of relative identity, all applying with equal right within this universe, will be given by different two-place predicables, e.g., ‘is the same token-word as,’ ‘is the same type-word as,’ ‘is the same dictionary-entry word as.’”
It is evident that Geach’s core thesis is that identity is relative and depends on the sortal noun A in “the same A.” Things within the same universe of discourse can be identical or non-identical under different standards of identity (i.e., different “same A” predicates), without the things themselves changing.
That is to say, Geach’s theory relies on a key commitment: when we switch from one standard of identity f1 (e.g., “same ship as material object”) to standard f2 (e.g., “same ship with causal history”), the individuals x and y under discussion themselves remain unchanged. That is, the old ship x and new ship y are judged identical under f1 and non-identical under f2, but the metaphysical constitution of x and y does not undergo any change due to the switch in standards.
Now, let us rigorously examine this commitment. In the Ship of Theseus example, let:
· x = the old ship (the object at t₁ constituted by planks M₁…Mₙ)
· y = the new ship (the object at t₂ constituted by planks M₁’…Mₙ’, with all materials replaced)
When we examine under standard f1 (“same material ship”), the attribute set of x and y is A_f1 = {constituted by planks, having the macroscopic shape of a ship, …}. Under this attribute set, x and y are indistinguishable (assuming the planks correspond one-to-one identically), so according to the standard of f1, x ≡ y.
When we examine under standard f2 (“same ship with historical continuity”), we actually introduce a new attribute set A_f2 = {constituted by planks, having the macroscopic shape of a ship, …, **having a continuous historical path from t₁ to t₂**}. The problem is: does the original x possess all the attributes in A_f2? Clearly not, because x at time t₁ has not yet acquired the historical path from t₁ to t₂—historical path is a relational property added after the fact, and it is not part of x’s intrinsic constitution in isolation. Thus, the “old ship entity” judged under f2 is actually no longer the original x, but a composite x’ = (x, history_path) associated with t₂. Similarly, the new ship y under f2 is regarded as y’ = (y, history_path), but y’s historical path differs from x’s.
We thus see a decisive difference: the pair of entities compared under f2 is not the original (x, y), but (x’, y’). And x’ ≠ x (as attribute sets), y’ ≠ y. Geach asserts that “the things themselves have not changed,” but strictly speaking, when we switch identity standards, we have quietly replaced the entities under discussion—because the applicability of each standard itself presupposes different attribute packages. This is not the same batch of things exhibiting different identity relations under different standards, but different sets of things naturally yielding different conclusions under different judgment frameworks.
Consider, for example, when we discuss whether Superman is a human or a superhuman. When judging whether he is human, we use his human attributes for comparison. At this point, he only needs to satisfy human attributes (at this point he is A); whether he is an ordinary person has no effect, because we are looking at his human attributes. But when we ask if he is a superhuman, we compare his superhuman attributes (at this point B) and not just his human attributes. Here A is not equal to B; A is A, B is B. A and B may overlap, but because their boundaries differ, they are not actually the same thing.
Assume there exists an object O.
When we discuss: Is O a human?
We actually focus only on the “set of human attributes.”
Thus we obtain: A = O as an object under the human boundary.
When we discuss: Is O a superhuman?
People focus on: “the set of superhuman attributes.”
Thus we obtain: B = O as an object under the superhuman boundary.
But A ≠ B
Because:
The constitutive boundary of A is the set of human attributes;
The constitutive boundary of B is the set of superhuman attributes;
Although they largely overlap, they are not the same object.
This has a subtle difference from ordinary relative identity, because Geach’s general idea is: the same object, under different standards, yields different identity judgments.
This paper holds: when the standard changes, the object changes.
Let:
A = {human attributes}
B = {human attributes + superhuman attributes}
Then: A ⊂ B
But: A ≠ B
Therefore: the object defined by A and the object defined by B are not the same object.
This reasoning is itself self-consistent, but most philosophers might question the first step: why is an object defined by its attribute set? Rather than: the object exists first, and attributes attach to it afterward?
When an entity interacts with the world, we call the features left behind its attributes. If an entity interacts with the world without producing any features—that is, without any attributes—then its existence has no explanatory relevance to us, like the dragon in the garage.
Thus, it is not that attributes determine the existence of matter, but that the boundary of observable matter is the boundary of attributes. The “object” here is no longer the thing-in-itself in the Kantian sense, but an object incorporated into the cognitive and explanatory system. Therefore, this paper does not claim that objects must be defined by attributes, but that objects defined by attribute sets have significance for explanation; otherwise, they are redundant in explanation (see section 3.7).
Therefore, this paper holds that the “relativity” presented in Geach’s theory is an illusion produced by category confusion. He correctly pointed out that A in “same A” is crucial, but he erroneously assumed the existence of a “thing itself” that can freely shuttle between different A’s while remaining unchanged and retaining explanatory significance.
2.4. Conservation
2.4.1. The Problem of the Indistinguishability of Identical Particles in Quantum Mechanics in Philosophy Poses the Most Severe Challenge to Leibniz’s PII, yet it Provides a Natural, Physically Evidenced Model for this Theory
Current philosophical discussions of identical particles, facing the challenge quantum identical particles pose to PII, are mainly divided into two types of solutions: revisionist and revolutionary. The former attempts to salvage some form of the principle of individuality, while the latter abandons individuality itself.
Saunders’ scheme undoubtedly represents one of the most ingenious and technically rigorous attempts in the revisionist path. Through his subtle definition of “weak discernibility,” he successfully liberates the discussion from the dead end of intrinsic properties and provides a creative perspective for seeking individuation foundations in relationality. The complexity of this approach and the extensive discussion it has provoked itself prove its profound philosophical value. However, it is precisely this technical complexity that exposes a potential cost of the scheme: its definition of “purely extensional relational properties,” though striving for precision, inevitably introduces considerable terminological vagueness, such that even its defenders must carefully handle accusations of circular argumentation (Muller & Saunders, 2008). More centrally, the entire theoretical edifice of the scheme is built upon an unsettling presupposition: that the “individuality” of identical particles must, and can only, be “salvaged” by finding some (even relational) individuating property. This makes its theoretical efforts—no matter how ingenious—essentially a kind of ad-hoc patching to salvage a premise. When applied to states with indeterminate particle number in quantum field theory, this strategy of constantly introducing new relational properties to salvage individuality becomes increasingly ad-hoc: it no longer seems like an elegant inference from the theory itself, but more like an increasingly expensive cost paid to maintain the theoretical premise (that individuality must exist).
Facing the difficulties of revisionism, another revolutionary approach chooses a more radical path. Scholars represented by Décio Krause (2011) have put forward a highly subversive argument: quantum particles may not be “individuals” in the traditional metaphysical sense at all. Therefore, laws based on individual identity were misapplied from the beginning. They should be understood as “non-individuals” and require highly specialized mathematical tools such as quasi-set theory to describe.
Krause’s scheme is notable for its conceptual thoroughness and consistency; it wholeheartedly embraces the most counterintuitive features of quantum mechanics and decisively breaks with our entire classical framework of objects and spatiotemporal localization. This resolute stance is undoubtedly clean and efficient in theory. However, the corresponding cost of this efficiency is that the concept of “non-individual” itself creates considerable explanatory burdens in metaphysics, requiring us to abandon a whole set of deeply rooted intuitive understandings of what “one thing” means.
2.4.2. Solution: An Attribute Set
The above two approaches share a deep misunderstanding: both attempt to find answers to a wrongly posed question. The problem is not “What is the correct individuating property?” but “In what category are we asking the identity question?”
This paper provides a framework for this. We define an observable particle as: a particle = (particle attributes, coordinate), where the particle attribute set (mass, charge, spin, etc.).
· When we inquire under the domain (particle) = particle attributes, i.e., comparing only intrinsic attributes, all identical electrons are (electron) = (mass m_e, charge -e, spin 1/2...). According to Axiom 1, they are indeed the same electron e in this domain. This explains the root of their indistinguishability.
· When we inquire under the domain (particle) = (particle attributes, coordinate), since coordinates are necessarily different, (particle₁) ≠ (particle₂), therefore they are different particle states. This explains why we observe multiple scattering events in experiments.
Therefore, the confusion brought by quantum identical particles stems from erroneously substituting the difference in coordinate attributes into the judgment of identity for particle attributes. This paper resolves the contradiction by clearly distinguishing these two attributes: they are both “one” (as logical concepts) and “many” (as manifestations in specific spacetime). Particle annihilation and creation merely represent the decoupling and re-coupling of the e with different coordinates.
This scheme absorbs the advantages of Krause’s scheme in acknowledging quantum peculiarities (by interpreting “non-individuality” as identity in particle attributes) while avoiding its radical metaphysical costs (we are still talking about “quanta,” just in different categories); at the same time, it explains why Saunders’ strategy of introducing relational properties seems feasible in some cases (because he erroneously treated coordinate attributes as individuating bases for particle attributes) yet fundamentally went astray.
2.4.3. Formal Derivation of Conservation:
Assume a basic particle state can be expressed as: particle = (particle attributes, coordinate), where:
· Particle attribute set (e.g., mass m, charge q, spin s)
· Coordinate set (e.g., position x, time t).
Formalization:
1. When the particle’s domain is particle = (particle attributes, coordinate₁), meaning an electron at a certain coordinate.
2. Coordinate decoupling (destruction): (particle attributes, coordinate₁) → (particle attributes), (coordinate₁) ⇒ The particle degenerates into a pure eigenstate (particle attributes). Due to lack of observable basis (coordinate = ∅), it is unmeasurable. ∀ particles (particle attributes, coordinate₁) and (particle attributes, coordinate₂), it can be found that: (particle attributes) ≡ (particle attributes) indicates:
· When two particles’ eigen-attributes are indistinguishable (particle attributes ≡ particle attributes), regardless of how their spatiotemporal coordinates differ (coordinate₁ ≠ coordinate₂), the particle is the same electron e = (particle attributes) projected in different spacetimes.
Physical interpretation:
· Particle annihilation ⇨ Set decoupling rather than annihilation ⇒ e = (particle attributes) becomes a concept.
· Particle creation ⇨ The same e binds to coordinate₂ ⇒ Observed as reappearance. Example: Electron e disappears at coordinate₁ and appears at coordinate₂, which is actually the coordinate migration of electron e = (q=-1e, m_e, s=1/2...): (e, coordinate₁) → (e) → (e, coordinate₂). Its electron identity is guaranteed by n ≡ n.
Direct corollary: Conservation laws, i.e., what logic permits exists, will neither annihilate nor be updated.
2.5. Symmetry
Max Black’s (1952) thought experiment of a symmetric universe poses the most extreme challenge to Leibniz’s strong PII principle. He imagines a universe containing 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, mutual symmetry). Black argues from this that this is a real scenario of “two” things, thereby refuting PII—that there is no attribute that can distinguish them, yet they are still numerically distinct entities.
Traditional response strategies are mainly divided into two types: one questions the metaphysical possibility of such a symmetric universe (e.g., requiring a basis for “numerical difference” itself, which usually returns to some hidden attribute); the other, like Saunders (2003), argues that relational properties (such as “X miles from one sphere”) can themselves serve as weakened distinguishing bases. However, the former is criticized as ad-hoc, while the latter is difficult to work in Black’s original setup, because each sphere’s relational properties (“X miles from the other sphere”) are still completely identical.
This paper argues that the difficulties of Black’s challenge and traditional responses jointly stem from an unexamined presupposition: that “numerical twoness” is a primitive, irreducible fact. This theory provides a completely new analytical perspective. Under the framework of this paper, we must first clarify the domain of (n).
· If we define (sphere) as the set of all traditional attributes (intrinsic + relational), i.e., (n) = {mass M, spherical shape, ..., distance X from one sphere}, then according to the axiom, because (n) ≡ (n), we necessarily conclude sphere ≡ sphere. This seemingly directly yields the PII conclusion that Black sought to refute.
· However, Black’s intuition—”there are obviously two spheres here”—is not entirely without basis. This theory explains it as a mental fixation. The reason observers report “seeing two” is that their perspective itself is embedded in this symmetric spatiotemporal coordinate system. This paper argues that a fundamental error shared by Black and his commentators lies in defaulting that the referents of “sphere” and “sphere” necessarily correspond to two entities with independent spatiotemporal coordinates. This presupposition leaves them with only 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 whose attribute set is 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, the sphere being opposite itself is not a grammatical error but an accurate description of a singular coordinate topology. The visually presented “two” spheres 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, but rather the topological properties of space itself).
· System S: (S) describes the state of a single sphere bound to a special self-referential coordinate topological structure: (sphere, R_self-facing).
· Paradox dissolution: Black’s error lies in erroneously inferring from the system state (sphere, R_self-facing) the existence of two spheres (sphere_1, sphere_2). He confused categories, using the descriptive result of (S) to answer a question about (single sphere). In fact, a second sphere never existed; there has always been only one sphere, situated in a special coordinate topology that produces a “double-image projection.”
However, some may say that the number 2 is a primitive fact.
But consider: suppose there exist two completely indistinguishable things, but as soon as symmetry is broken they are no longer indistinguishable. Any third party that creates asymmetry can directly prove that they are not the same in a global sense, which means distinguishable items have appeared. Of course, if we say global symmetry, then it becomes Black’s symmetric universe. The solution is as in section 2.5; essentially, it is not two. Therefore, the description “there exist two indistinguishable things” is contradictory. Assume there exist two objects A and B. At the same time:
A and B have completely identical attributes;
There are no distinguishable items;
No positional differences;
No relational differences;
No historical differences;
No third-party observer can distinguish them.
Then: What exactly distinguishes A and B?
If one says: “They are just numerically different.”
Then you have actually introduced a new property:
A possesses the property of “being A rather than B.”
B possesses the property of “being B rather than A.”
Thus: They have already become distinguishable.
Conversely, if there are no distinguishable attributes, then it can only be a symmetric state, but once it enters the symmetric state, it means it has undergone self-involvement with itself.
Therefore, once there are no distinguishable attributes, their numerical distinction loses its existential basis. Thus, the expression that they exist as two requires stronger argumentation, because a number without basis is not a number but an artificial label.
Some may say that numerical difference itself is the basis; the world is just like that.
This leads to a question: If indistinguishable things can be more than one thing, then what is it itself? Is it identical to itself? If it is identical to itself, yet there can exist other things identical to it but not identical to it?
Let: A and B are completely identical.
Then: A = A holds.
Why?
Because of the law of self-identity.
Then why does A ≠ B also hold?
If there is no difference whatsoever between A and B.
Then: Where does the fact of “inequality” come from?
Therefore, this paper holds that numerical multiplicity must have a distinguishable structure as its basis; “two” without basis is merely artificial naming, not additional existence.