The Limits of the Self in Ontological Materialism
On
the Indeterminacy of Exclusive Personal Identity
Abstract
This paper examines whether a strictly materialist worldview is capable of grounding exclusive personal identity over time. The point of departure is the distinction between qualitative identity and numerical identity, together with the observation that many contemporary debates tacitly assume that one’s own experiencing self continues as exactly the same subject. By means of a copying thought experiment, and by taking into account the continuous material turnover in the human brain, the paper shows that, under materialist assumptions, qualitative duplicability is in principle possible. In such parity scenarios, all functional, causal, and psychological criteria of identity are satisfied multiple times, without yielding any criterion that fixes exclusive numerical identity.
The resulting problem is neither epistemic nor technical, but structural: materialism cannot derive exclusive personal identity from material facts alone. The analysis makes explicit the following combination of premises: (i) ontological materialism, (ii) the principled duplicability of material–functional structures, and (iii) the exclusivity of personal continuation. Under these assumptions, exclusive numerical identity cannot be secured by additional data or technical detail, but only by revising the underlying claim or by introducing a non-qualitative supplementary principle (for example, a primitive identity fact).
Alternative theories of personal identity are not refuted, but classified as coherent strategies of claim revision that abandon exclusivity. The paper concludes that the issue at stake is not an empirical gap, but a systematic limit of materialist ontologization. Materialism is therefore not challenged as a scientific method, but its claim to fully explain the self as an exclusively identical subject is thereby constrained.
Table of Contents
1. Introduction: The Problem of Identity in the Materialist Conception of the Self 3
2. Conceptual and Premise Clarification: Materialism, Identity, and Exclusivity 5
2.1 Methodological and Ontological Materialism 5
2.2 The Self and Personal Identity 5
2.3 Exclusivity of Personal Continuation 6
2.4 Clarification: Exclusivity and Indexical Identity 6
2.5 substrate continuity as an Implicit Assumption 6
2.6 Methodological Set-Up of the Investigation 7
3. Time, Process, and the Non-Punctuality of the Self 8
3.1 The Instant as an Idealized Limiting Concept 8
3.2 Consequences for a Materialist Conception of the Self 8
3.3 Processuality and Identity Attribution 8
3.4 Initial Consequences for Memory and Continuity 9
4. The Copy Thought Experiment and the Parity of Personal Identity 10
4.2 Qualitative Equality and Numerical Indeterminacy 11
4.3 The Indexical Dimension of the Problem 11
4.4 substrate continuity as an Apparent Solution 12
5. Material Carrier, Metabolism, and the Fragility of Numerical Identity 13
5.1 Material Turnover in the Brain 13
5.2 substrate continuity and Its Implicit Interpretation 13
5.3 The Gradual Emergence of Parity 14
5.4 The Sorites Effect and the Absence of a Threshold 14
6. Alternative Theories of Personal Identity and Their Limits within a Materialist Framework 16
6.1 Psychological Continuity 16
6.3 Perdurantism and Stage Theories 16
6.5 Functional and Information-Theoretic Models 17
7. The Systematic Limit of Ontological Materialism 18
7.1 The Incompatibility of Central Commitments 18
7.2 The Status of the Identity Criterion 18
7.3 The Indexicality of the Self 19
7.4 Consequence: Revision of the Claim or Metaphysics 19
8. Conclusion: The Self and the Limits of Materialism 20
Personal Identity, Exclusivity, Fission 21
Stage Theory, Perdurantism, Four-Dimensionalism 21
Functional, Narrative, and Information-Based Models of the Self 21
Metabolism, Material Instability, Brain 21
Time, Presentness, Temporal Extension of Experience 21
Realism, Ontology, Epistemological Limits 22
Self, Observer, Second-Order Observation 22
Appendix A: Epistemic Preconditions of Materialist Self-Description 23
The question of personal identity belongs to the fundamental problems of philosophy of mind and metaphysics. In everyday life as well as in scientific contexts, it is usually taken for granted that a person remains the same over time, despite bodily, psychological, and biographical change. This assumption structures responsibility, self-concern, life planning, and personal relationships. It is rarely made explicit or justified, but functions as an implicit background condition.
In contemporary discourse, this assumption is often understood within the framework of a materialist worldview. Materialism, in its various forms, holds that everything real is ultimately reducible to physical processes. The self, consciousness, and personal identity are accordingly treated as products or properties of material carriers, in particular the brain. The success of the natural sciences has endowed this conception with considerable plausibility; materialist assumptions often appear no longer as a metaphysical position, but as a natural extension of scientific insight.
The present investigation is not primarily directed against elaborated specialist theories of personal identity, but against a widespread everyday materialist claim. In practical contexts of life planning and self-concern, it is typically presupposed that precisely this experiencing subject continues to exist in an exclusive manner, even though this claim is often theoretically relativized while remaining practically operative.
It is precisely this proximity to scientific thinking that makes a conceptual clarification necessary. For materialism appears in two clearly distinguishable forms: as a methodological principle that operates with physical causes and models, and as an ontological thesis that claims that only material entities exist. While methodological materialism is indispensable for the natural sciences, ontological materialism advances a stronger claim: it purports to answer questions of identity, persistence, and the self without recourse to metaphysical supplementary assumptions.
This paper addresses this latter claim. It does not ask whether materialism is successful as a scientific method, which is taken for granted, but whether an ontologically understood materialism is capable of coherently grounding the exclusive persistence of one’s own experiencing self over time. In doing so, it makes explicit a claim that is usually left implicit in everyday practice: the assumption that not merely some functionally or psychologically equivalent system continues to exist, but that precisely this experiencing subject does so.
The central thesis is the following:
Under the assumption of strict materialism combined with a claim to exclusive personal continuation, the identity of the self enters into a structural tension. Neither functional continuity nor material carrier assumptions suffice to secure numerical identity over time without the introduction of additional metaphysical commitments.
To assess this thesis, the paper proceeds in a systematic manner. First, the relevant concepts and premises are clarified with precision. It is then argued that the self, even within a materialist framework, cannot be understood as point-like, but must be conceived as a temporally extended process. On this basis, copying and parity thought experiments are analyzed in order to make the problem of exclusive identity explicit. Empirical findings concerning material turnover in the brain are subsequently introduced to assess the force of carrier-based identity arguments. Finally, alternative theories of personal identity are discussed and their status within a materialist framework is determined.
The aim of this investigation is neither to reject the natural sciences nor to establish a competing ontology. Rather, it is an internal consistency test: if ontological materialism claims to fully explain the self, it must show how exclusive personal identity can be secured without metaphysical supplementation. If this cannot be done, the result does not mark a weakness of science, but a limit of its ontological scope.
The paper does not develop a new theory of personal identity, nor does it defend an alternative ontology. It is intended as an internal consistency analysis of ontological materialism under an explicitly articulated claim to exclusive personal continuation. Systematically, it (i) explicates this claim to exclusivity, (ii) confronts it with the principled duplicability of material–functional structures, and (iii) shows that the resulting identity parity is not epistemic, but structural in nature. In this way, a limit of ontological materialization is identified without calling methodological naturalism or empirical research into question.
In order to meaningfully assess the question of personal identity within a materialist framework, a precise clarification of the underlying concepts and assumptions is required. Many misunderstandings in the debate do not arise from substantive disagreement, but from implicit shifts between methodological, ontological, and everyday-practical levels. This chapter therefore makes explicit the central premises under which the following analysis proceeds.
A strict distinction must first be drawn between methodological and ontological materialism.
Methodological materialism denotes a scientific working principle. It confines itself to explaining phenomena by means of physical causes, processes, and models. This approach does not make ultimate claims about being as such, but operates with idealized concepts, approximations, and measurement procedures that have proven effective in empirical practice. As a method, materialism in this sense is neither metaphysically demanding nor problematic.
Ontological materialism, by contrast, is a metaphysical thesis. It claims that only material entities exist and that all phenomena—including consciousness, the self, and identity—are fully reducible to material carriers and processes. This thesis goes beyond scientific methodology and advances a claim to a complete description of reality.
The present investigation is directed exclusively at this ontological reading. Methodological materialism is not criticized, but presupposed.
The concept of the self is used inconsistently in philosophical discourse. For the purposes of this paper, it is crucial to distinguish between different notions of identity.
Qualitative identity refers to sameness in properties. Two entities can be qualitatively identical without being numerically identical, as in the case of two perfectly matching copies of an object.
Numerical identity denotes sameness in the strict sense: one and the same individual persists over time. For the question of the self, only this form is relevant. The statement “I am the same person as yesterday” does not claim mere similarity, but strict numerical identity.
Personal identity, in this context, refers to the persistence of precisely this experiencing subject over time. It is not merely a third-person attribution, but possesses an ineliminably indexical dimension: the issue is not whether some subject exists, but whether this subject continues to exist.
A central, often unarticulated component of personal identity is the claim to exclusivity. In everyday life, it is tacitly assumed that exactly one future instance can be my continuation. This claim is stronger than mere psychological or functional continuity. It excludes the possibility that several equally valid continuations could exist simultaneously and yet all be “me.”
This exclusivity claim is not a theoretical idiosyncrasy, but structures fundamental forms of practice: responsibility, self-concern, future planning, and fear of one’s own death. Many materialist positions implicitly rely on it without explicitly addressing it.
In what follows, this claim is not taken for granted, but used as a test case: can ontological materialism explain how exclusive personal continuation is possible without introducing additional metaphysical assumptions?
The exclusivity claim employed in the following analysis is stronger than the mere requirement of functional or psychological continuity. What is meant is not only that personal continuation should be uniquely assignable, but that it must in principle be excluded that several equally valid continuations exist that are all equally numerically identical with the original self.
This exclusivity has an ineliminably indexical dimension. The question of personal identity does not concern only the third-person perspective (“which instance counts as the continuation?”), but also the first-person perspective (“which of these possible continuations am I?”). The following argument aims primarily at the structural incompatibility between duplicability and exclusivity within an ontologically materialist framework; the indexical dimension is invoked to show why this incompatibility must not be misunderstood as a mere problem of attribution.
Within a materialist framework, it is natural to bind numerical identity to the continuity of a material carrier. The intuitive assumption is that the self remains identical as long as the material carrier—most notably the brain—continues along a continuous spatiotemporal worldline.
At this point, a conceptual distinction is decisive. Material substrate continuity in the strict sense refers to the persisting numerical identity of the same material substance along an uninterrupted worldline. This must be distinguished from the mere continuity of functional or structural organization, which can in principle be preserved even under complete material replacement and is therefore duplicable. An unmarked transition between these levels shifts the identity criterion without justification, replacing material persistence with functional equivalence while continuing to speak in the language of numerical identity.
This assumption is by no means trivial. It presupposes that material continuity can be clearly determined and that functional equivalence without substrate continuity is insufficient to secure identity. At the same time, it is often overlooked that material carriers themselves are temporally extended processes and do not satisfy point-like identity criteria.
The following analysis distinguishes between three analytically distinct levels:
functional continuity,
material substrate continuity,
exclusive numerical identity.
Only by drawing these distinctions does it become visible where the materialist concept of identity comes under pressure.
Finally, the methodological set-up of this investigation should be made explicit. The argument operates as a conditional test. It does not ask which of the premises should be rejected, but whether they can be jointly maintained without additional metaphysical commitments.
If
(a) ontological materialism is true, and
(b)
exclusive personal continuation is claimed,
then it must be shown how numerical identity can be secured without metaphysical supplementation.
The paper does not claim to refute all possible notions of identity. It specifically examines whether the widespread materialist self-conception—as implicitly endorsed in everyday life, popular science, and parts of philosophy—is internally consistent.
Clarifying note for systematic readers (axiomatic framing)
The following investigation does not operate with an empirical hypothesis, but with an explicit combination of premises whose internal coherence is examined. The argument is therefore to be understood as a structural consistency analysis.
For the remainder of the paper, three assumptions are adopted:
Ontological materialism
All that is
real is fully determined by material or physical facts. There are no
non-physical carriers of personal identity.
Duplicability of material–functional
structures
Material and functional configurations are
in principle copyable. This assumption concerns logical possibility,
not technical feasibility.
Exclusivity of personal continuation
Personal
identity is understood in the strong sense: exactly one future
instance can be the continuation of the currently experiencing
subject.
These assumptions are individually widespread in contemporary debate, but are rarely made explicit jointly. The aim of the investigation is not to refute any of them, but to test whether they are mutually consistent without recourse to additional metaphysical principles.
Approaches that explicitly abandon the exclusivity claim (for example, relational or purely functional accounts of identity) are not criticized in what follows, but classified as coherent strategies of claim revision. The argument is directed exclusively at positions that seek to retain exclusivity.
A central assumption in debates about personal identity—often left implicit—is the idea that the self can be determined at a particular point in time. Identity is frequently conceived as a relation between discrete temporal points: a self at t₁, the same self at t₂. While this conception appears intuitive, closer examination reveals it to be problematic, especially within a materialist worldview.
In the physical description of the world, the instant plays a formal role, but not as an independently substantive ontological unit. Measurements always possess a finite temporal extension, physical processes unfold continuously, and state descriptions refer to temporal intervals. The exact instant thus functions as a mathematical limiting concept rather than an independently instantiated ontological unit.
For the question of personal identity, this does not imply that point-like state descriptions are meaningless, but rather that they lack independent ontological carrier status. Identity therefore cannot be reduced to the equality of isolated temporal points, but must be understood in terms of temporal relations and transitions.
If ontological materialism is taken seriously, it must accommodate this structure. For questions of personal identity, a self that is fully materially realized cannot be a point-like object, since identity cannot be determined as equality at isolated instants, but only across temporal relations. A point-like self would be bound to an instant that itself is merely an idealization.
It follows that even within a strictly materialist framework, the self can be adequately described for identity purposes only as a temporally extended process. This processuality is neither a phenomenological assumption nor an introspective speculation, but follows from the conditions of materialist description itself.
This fundamentally shifts the identity question. If the self does not exist at a point in time, identity cannot be determined as point-wise equality. It must instead be grounded in relations, transitions, and structures within a temporally extended trajectory.
A temporally extended self is not a static object, but an ongoing process of organization and transition. Processes cannot be identified by being “paused” and compared; they can only be identified through their internal coherence, structure, and causal transitions.
In everyday experience, this processuality is usually obscured by a high degree of functional stability. Memories, character traits, and dispositions create the impression of a stable core. Yet this stability is already an achievement of temporal integration, not its precondition.
Identity thus ceases to be a given property and becomes an attribution extended over time. It presupposes connection rather than simultaneity.
The necessary temporal extension of the self has immediate consequences for the understanding of memory. Memory is not a later addition to an already existing self, but the minimal condition under which a persisting self can be ascribed at all. Without the capacity to integrate earlier states into present processes, there would be no self, but merely a sequence of unconnected states.
Within a materialist framework, memory is therefore not optional, but structurally necessary. It establishes the linkage between temporally separated segments of a process and thereby enables the attribution of numerical identity.
This insight will be decisive in what follows, especially in the analysis of copying and parity scenarios. For once multiple temporally extended processes share the same functional structure and the same memory base, the question arises whether and how exclusive identity can still be meaningfully grounded.
The attribution of numerical identity is thus not an empirical finding, but a practical act: we presuppose continuity in order to act, plan, and assume responsibility. This presupposition is functionally indispensable, even if it cannot be established as an objective fact within a materialist framework.
Within a materialist framework, the self cannot be understood as a point-like object. The instant itself is not an ontological element of physics, but an idealization. A materially realized self is therefore necessarily temporally extended and processual.
This already reveals a first boundary: the identity of the self cannot be reduced to equality at discrete temporal points. It must be explained in terms of temporal relations. Whether and how this is possible under the assumption of exclusive personal continuation remains open and constitutes the subject of the following chapters.
In order to sharpen the question of exclusive personal identity within a materialist framework, a thought experiment is useful that holds all functional, psychological, and causal factors constant while nevertheless forcing an identity decision. Such thought experiments do not serve as technological predictions, but as tools of conceptual clarification: they test which criteria actually carry identity. The copy thought experiment has no probative function in the empirical sense, but serves to bring into sharp relief a problem that already follows from the structure of materialist identity criteria themselves, namely the absence of an exclusive identity marker.
Suppose a human being is copied while in a state of complete unconsciousness. The reproduction affects all functional and physical properties of the brain that are relevant for consciousness, memory, self-ascription, and action-dispositions. The original system remains intact. After the copying procedure, two physically separate systems exist that coincide in all identity-relevant respects.
The copy does not consist of the same atoms as the original system, but of numerically distinct atoms that realize the same functional organization with respect to the relevant properties and relations. It is not assumed that the two instances are fully identical in every physical detail. What matters is solely that both systems coincide with respect to all materialist criteria typically invoked for personal identity. Any further differences are irrelevant to identity attribution and therefore do not resolve the parity.
The spatial positioning of the two instances is subsequently determined by a random mechanism, such that neither from the internal perspective of the individuals involved nor through later observation can it be reconstructed which position was associated with the original material carrier.
Both instances awaken. They possess the same memories, the same character, and the same self-conception. Both are aware of the copying experiment and fully understand that they have been duplicated at the relevant functional level. They know that all identity-relevant properties are symmetrically distributed.
For this very reason, both instances recognize that they cannot, in principle, determine whether they are the original person or the copy. What is lacking is not information about the world, but an identity criterion capable of breaking the symmetry. No internal state, no memory, and no material or functional feature available within the materialist descriptive framework allows either instance to determine its own exclusive identity. Each also recognizes that the other can, with equal justification, claim to be the continuation of the original self.
This scenario is logically consistent and does not conflict with any materialist basic assumptions. Whether it is technically realizable is irrelevant for the following argument. What matters is solely that ontological materialism contains no principled barrier against identity-relevant duplicability and provides no criterion that could fix exclusive personal identity in such parity cases.
In the copy scenario, all conditions typically relied upon by materialist accounts of identity are satisfied: complete psychological continuity, causal embeddedness in the same biographical trajectory up to the point of copying, functional organization of the conscious process, and a physical realization that is equivalent for all identity-relevant purposes. It is precisely this completeness that renders the case conceptually sharp.
For under these conditions, given the jointly assumed duplicability of material–functional structures and the claim to exclusivity, what arises is not merely uncertainty about some hidden detail, but a structural symmetry: the entire set of relevant facts is equally distributed across both instances. As a result, there is no difference within the materialist descriptive vocabulary that could fix exclusive numerical identity. The question of whether “this self” continues to exist in the strict sense lacks a determinable truth value in parity situations, because the framework contains no materially or functionally specifiable criterion capable of yielding a unique assignment under symmetric conditions.
This indeterminacy does not affect only the internal perspective of the individuals involved. From an external standpoint as well, no identity-deciding feature remains within a purely qualitative-functional materialist vocabulary that could fix exclusive numerical identity in symmetric parity situations. Any difference between the two instances would amount at most to a historical labeling, not to an ontologically effective distinction in the sense of exclusive continuation.
The copy case thus shows that materialist criteria reliably capture qualitative and functional continuation, but do not suffice to fix exclusive numerical identity in parity situations. It is precisely here that the central tension arises, which will be specified in the next section as an indexical problem.
The copy scenario does not concern merely an external attribution problem, but an ineliminably indexical dimension: the issue is not whether a continuation exists, but which continuation I am. This first-person perspective cannot be supplemented by further objective facts. All objectively describable properties are already fixed. The question of which continuation I am presupposes that numerical identity persists as exclusive sameness and merely needs to be located, a presupposition that is no longer available within a strictly materialist framework.
This makes clear that the problem is not epistemic, but ontological. What is lacking is not knowledge, but a decision-capable identity criterion that could be specified in material or functional terms. This is not a mere epistemic deficiency, but the absence of a principled identity criterion within the materialist descriptive vocabulary itself. Materialism can describe that two continuations exist, but it cannot explain why exactly one of them should be the exclusive continuation of the original self.
This indeterminacy is not confined to technical thought experiments. Even in ordinary waking, we tacitly presuppose that we are numerically the same self as before, without possessing, within a strictly materialist framework, any unambiguous identity marker.
A natural reaction consists in binding exclusive identity to the continuous worldline of the original material carrier. On this reading, the original would be the continuation, while the copy would merely be a new person with an identical initial state.
A related objection holds that exclusive personal identity should not be tied to qualitative or functional equality, but to the causal-historical continuity of a concrete material process. On this view, the instance that arises along the same spatiotemporal worldline would be the continuation of the original self, regardless of functional duplicability.
Such a criterion is, in principle, formulable within a materialist worldview. However, it shifts the concept of identity from a sameness derivable from qualitative facts to a genealogical stipulation. It thereby answers the third-person question (“which instance counts as the continuation”), but not the indexical question of why exactly this continuation should be the exclusive continuation of the experiencing subject, given that all experience- and function-relevant properties are symmetrically distributed.
This solution therefore displaces the problem without resolving it. For it presupposes that substrate continuity itself is an ontologically privileged criterion. This assumption is not grounded in functional or psychological considerations, but constitutes a metaphysical supplementary commitment. Moreover, the copy scenario precisely shows that substrate continuity is not an accessible property for the experiencing subject. Both instances share the same memory base and cannot distinguish their respective carrier histories from within experience.
Exclusive first-person attribution thus remains unjustified even when substrate continuity is invoked.
The copy thought experiment shows that qualitative and functional criteria are insufficient to secure exclusive personal identity. As soon as multiple equally valid continuations exist, a parity arises that ontological materialism cannot resolve without introducing additional metaphysical principles.
This result is independent of whether one considers the scenario technically plausible. It follows solely from the assumption that the self is fully determined by material and functional properties. If these properties are duplicable, exclusive identity is no longer uniquely determinable.
In the next chapter, it will be shown that this problem does not arise only in hypothetical copy scenarios, but is already structurally present in the real biological process of material turnover, especially in the brain.
The copy thought experiment has shown that exclusive personal identity cannot be uniquely determined in parity situations within a materialist framework. This result might appear purely hypothetical as long as copying is regarded as technically unrealistic. Yet the underlying problem is not confined to thought experiments. It also arises in the real biological process of material turnover, especially in the brain, which materialism treats as the carrier of the self.
The human organism is subject to continuous metabolism. Molecules are constantly replaced, transformed, or eliminated. This process does not affect only peripheral tissues, but also the brain. Water, ions, neurotransmitters, proteins, and lipids are renewed on different temporal scales. Even in post-mitotic neurons, the material composition does not remain stable, even though cellular structure as such is preserved.
Empirical studies show that within a few years, a large portion of the molecular substance of the brain has been replaced at least once. Functional organization may remain largely stable, but the material carrier in the strict sense does not. The brain is not a static object, but a dynamic system undergoing continuous material change.
According to current knowledge, the molecular substance of the brain undergoes ongoing turnover over the course of a lifetime, while functional organization remains largely stable. The copy case is therefore not a categorical special case, but a limiting case of the same structural separation between material substance and functional organization.
To illustrate this lack of a material core, it suffices to consider those brain structures that are regarded as particularly long-lived. Even post-mitotic neurons and their central molecular components, such as synaptic proteins or elements of the nuclear pore complex, are subject to slow but continuous material turnover. Their functional stability does not rest on substantial identity, but on dynamic renewal. Thus, even at this level, there is no clearly delimited material core that could function as the carrier of exclusive personal identity.
Within a materialist conception of the self, personal identity is often implicitly tied to what is called the continuity of the material carrier, without specifying whether this continuity is understood in a substantial or merely functional sense. On closer inspection, however, this continuity is usually understood not as substantial, but as functional. What is meant is not the preservation of concrete material constituents, but the persistence of a certain functional organization. In this way, the identity criterion shifts from material sameness to functional continuity without this transition being explicitly acknowledged.
This shift renders the carrier criterion unstable. If the material basis itself is interchangeable as long as organization is preserved, it can no longer serve as a non-duplicable foundation of exclusive identity. Material substrate continuity is then reduced to a pragmatic proxy for functional persistence rather than a genuine material identity criterion.
Conceptual clarification. The reference to continuous material turnover in the brain does not function in the present argument as an empirical refutation of a materialist identity criterion. Rather, it is intended to make visible a conceptual shift that often occurs tacitly in materialist self-conceptions: the reinterpretation of material substrate continuity in the strict sense as functional or organizational continuity. The metabolic findings illustrate that, in the real biological process, there is no clearly isolable material core that could function independently of functional organization as the carrier of exclusive identity. The argument is therefore directed not against empirical findings, but against an unacknowledged equivocation in the notion of a carrier.
Continuous metabolism does not produce an abrupt copy scenario, but a gradual replacement of the material carrier. For the identity question, however, this difference is irrelevant. What matters is that after a sufficiently long period of time, no material substance remains that previously constituted the carrier of the self. The same question therefore arises as in the copy case: in what sense is it still the same self?
The gradual replacement of the material carrier differs from the copy scenario not with respect to the underlying structure of the identity problem, but only in the temporal distribution of change. In both cases, material substance is replaced while functional organization persists.
Whether the material carrier is replaced step by step or all at once does not affect the logical core of the problem. In both cases, the issue is whether numerical identity is to be bound to persisting material substance or merely to functional continuity. If the latter is accepted, the carrier argument loses its ontological status and is reduced to a pragmatic attribution of functional sameness.
This consequence can be sharpened further. A significant portion of the material substance that once constituted a human brain leaves the body over the course of metabolism. Under purely materialist assumptions, it would in principle be possible to collect this discarded matter and reassemble it in its former configuration.
Such a system would be materially closer to the earlier self than the present organism, whose substance has largely been replaced. Yet hardly anyone would claim that one’s experience continues in this reconstruction. It is precisely here that the tension between material sameness and experienced continuity becomes particularly salient.
The copy problem thus turns out not to be an exotic limiting case, but an explicit sharpening of a problem that is already structurally present in the everyday biological process of metabolism.
A natural objection holds that material turnover is real but identity-irrelevant as long as it occurs gradually. This line of reasoning, however, implicitly presupposes a threshold: at some point, the exchange would have to be so extensive that identity is no longer preserved.
Such a threshold is neither empirically determinable nor conceptually stable. Any boundary drawn would be arbitrary and thus philosophically unusable. If identity does not cease at 98%, 99%, or 99.9% replacement, then there is no non-arbitrary point at which it does. What remains is the alternative of either fully detaching identity from the material carrier or binding it to a non-empirically accessible supplementary criterion.
Material turnover in the brain shows that, in the real biological process, no substantial material basis is preserved that could carry numerical identity. The appeal to substrate continuity therefore turns out either to be functional continuity under another name or to involve an additional non-qualitative stipulation. In neither case is exclusivity derived from material description.
The result reached so far can thus be condensed: neither functional continuity nor material carrier assumptions suffice to coherently ground exclusive personal identity within ontological materialism. The problem that becomes explicit in the copy scenario is already structurally present in the real self.
In the next chapter, alternative theories of personal identity will be considered in order to examine whether, and in what sense, they solve this problem or merely circumvent it.
In light of the difficulties involved in materially grounding exclusive personal identity, a variety of alternative criteria have been developed in philosophy. These theories aim to secure identity even when material substance changes or is functionally duplicable. The most influential approaches will be presented below and assessed in light of the preceding analysis.
Theories of psychological continuity define personal identity in terms of persisting mental states such as memories, beliefs, intentions, and character traits. What matters is not material sameness, but the causal connection between psychological states over time.
These approaches successfully capture many everyday intuitions. However, they reach a limit in copy and fission scenarios. As soon as two instances share the same psychological structure, both satisfy the criterion equally. Exclusive identity is thereby not secured, but is no longer treated as a necessary requirement. The theory explains what suffices for responsibility and rational agency, but not which instance is the continuation of the original self.
In its most radical form, this approach was defended by Derek Parfit. Parfit argues that personal identity is not a further fact beyond psychological continuity. What matters is a sufficiently strong relation of psychological connectedness, not numerical identity.
This position is internally consistent and philosophically influential. However, it does not solve the identity problem but eliminates it by explicitly rejecting the exclusivity claim. With respect to the question of whether precisely this experiencing subject continues to exist, the approach deliberately suspends any further fact beyond psychological relations.
Perdurantist theories understand persons as temporally extended entities composed of different temporal parts. Identity is here grounded either in belonging to the same four-dimensional object or in relation-based continuation relations between stages.
This approach also avoids the problem of exclusive identity by redefining it. In fission scenarios, multiple legitimate continuations arise without any one of them being privileged. The original problem is thus not solved, but neutralized by a modified structure of claims.
Animalism ties personal identity to the persistence of the same biological organism. Psychological continuity is secondary; what matters is biological unity.
At first glance, this approach seems to offer a clear answer. However, it comes into conflict with the copy and parity problem. Even if the biological organism serves as the identity anchor, it remains unclear from the perspective of the experiencing subject why precisely this organismic lineage should constitute the exclusive continuation. Moreover, it remains unexplained why functionally identical continuations should in principle be excluded once biological criteria are satisfied.
In functional or information-theoretic approaches, the self is understood as a pattern, process, or informational structure that can be realized on different material carriers. These models are particularly compatible with materialist and technological perspectives.
Their consequence is clear: if the self is a duplicable pattern, exclusive identity is no longer meaningfully formulable. Multiple instances can realize the same pattern without any of them being privileged. Continuation of the self is replaced by functional equivalence.
All alternative theories considered are internally consistent and philosophically serious. None of them, however, resolves the problem of exclusive personal identity within a materialist framework. Either they explicitly abandon the exclusivity claim, or they shift it to criteria that are not derivable from material or functional description.
In many of these approaches, it remains implicit to what extent the abandonment of exclusivity is consistently maintained with respect to everyday self-assumptions, future-oriented concern, and practical first-person attributions.
This confirms the result reached so far: the problem does not arise from a lack of theoretical diversity, but from a structural tension between materialism and the claim that precisely this experiencing subject persists over time.
In the next chapter, this tension will be brought together and identified as a systematic limit of ontological materialism.
The preceding analyses can now be consolidated into a general thesis. The problem of personal identity is neither a marginal phenomenon of particular thought experiments nor a deficiency of specific theories. It marks a systematic limit of ontological materialism itself, once it claims to explain exclusive personal continuation.
Three assumptions lie at the core of the materialist conception of the self:
Ontological materialism: Everything that is real is fully determined by material and physical facts.
Duplicability: Material and functional structures are in principle copyable.
Exclusivity: Exactly one later instance is the continuation of the original self.
The preceding chapters have shown that these three assumptions cannot be jointly maintained under the restriction to purely qualitative and functional identity criteria. If duplicability is allowed, parity situations arise in which multiple instances satisfy all materialist identity criteria equally. Exclusivity can then no longer be justified. If exclusivity is nevertheless asserted, an additional identity principle must be introduced that is not derivable from material or functional facts.
The tension described here is neither epistemic nor technical in nature. It could not be resolved even under idealized conditions of complete physical information, since in parity cases all relevant materialist criteria are already exhausted.
Materialist explanations operate with properties, structures, relations, and causal histories. None of these categories contains a criterion that could derive an exclusive first-person assignment from qualitative properties and relations alone. Criteria such as causal–historical continuity can fix exclusivity, but they do so by means of an additional genealogical stipulation, not by extending qualitative description. All describable facts are symmetrically distributed.
It therefore follows that exclusive personal identity is not a result of physical description alone, but requires an additional stipulation. Whether this stipulation is understood as a haecceitistic identity fact, a metaphysical carrier core, or an unanalyzable thisness is an open philosophical decision. In every case, it exceeds what ontological materialism can justify on its own resources.
This limit becomes particularly clear in the indexical structure of the first-person question. The question “Which of these instances am I?” does not point to a further objective fact, but to a self-location that cannot be externalized. It is neither measurable nor functionally substitutable.
Materialism can describe that experience occurs, that conscious processes are realized, and that psychological continuity is present. What it cannot explain is why a particular experience should exclusively continue this self. The first-person perspective remains as an irreducible remainder.
More precisely, it follows that within a strictly materialist framework it cannot be determined on the basis of any non-qualitative criterion which currently experienced self is numerically identical over time. This attribution is continuously enacted, but not accounted for.
The diagnosed limit thus does not concern experience as such, but the ontological projection in which the self is to be fixed as an exclusively identifiable object within a materially described world.
From this analysis, three consistent but clearly distinguishable options follow:
Revision of the claim: One abandons exclusive personal identity and accepts purely functional or relational continuation.
Metaphysical supplement: One augments materialism with a primitive identity fact that guarantees exclusivity.
Indeterminacy: One accepts that in parity cases there is no determinate answer as to who the continuation is.
None of these options is empirically enforceable. The choice between them is philosophical rather than scientific.
Ontological materialism reaches a principled limit in explaining exclusive personal identity. This limit does not arise from ignorance, technical impossibility, or empirical uncertainty, but from the structure of the explanatory framework itself. Exclusive numerical identity is not a property that can be reconstructed from material facts.
Result. Under the joint assumptions of ontological materialism, the principled duplicability of material–functional structures, and a strong exclusivity claim of personal continuation, numerical personal identity cannot be derived from material or functional facts alone without either introducing an additional non-qualitative identity principle or abandoning the exclusivity claim.
In the final chapter, it will be shown that this limit does not mark the end of the analysis, but the point at which the metaphysical scope of materialist conceptions of the self can be precisely determined.
The aim of this paper was to examine the viability of ontological materialism with respect to personal identity, insofar as it endorses the widespread claim that precisely this experiencing subject persists over time. The analysis has shown that this claim cannot be fulfilled within a materialist framework without additional assumptions.
The starting point was the distinction between qualitative and numerical identity. While functional, psychological, and causal criteria reliably capture qualitative sameness, numerical identity remains in principle indeterminate under conditions of possible duplicability. Thought experiments involving copying and fission make this indeterminacy explicitly visible without relying on technical speculation. They function as conceptual boundary tests of materialist identity criteria.
The inclusion of real biological metabolism, particularly material turnover in the brain, shows that this problem is not confined to hypothetical scenarios. Even in natural biological development, no stable material basis remains that could uniquely secure numerical identity. Appeals to substrate continuity therefore turn out either to be disguised functional argumentation or to involve a metaphysical supplementary assumption.
Alternative theories of personal identity confirm this result. Psychological, functional, perdurantist, and animalist approaches each provide internally consistent answers, but at the cost of revising the original claim. Either exclusivity is abandoned, or it is replaced by criteria that are not derivable from the materialist descriptive vocabulary. These approaches do not refute the diagnosis; they illustrate it. They solve the problem by redefining the original identity claim.
The result of this analysis therefore does not amount to a refutation of naturalism as a scientific method. On the contrary, it shows precisely where the scope of scientific explanation ends. The transition from functional description to exclusive sameness is not an empirical step, but a metaphysical one.
The central insight can therefore be formulated precisely: ontological materialism can coherently explain personal identity only if it either relinquishes the claim that precisely this experiencing subject persists exclusively over time, or introduces an additional non-qualitative identity principle. If the exclusivity claim is maintained, such an additional principle is required that does not follow from material or functional facts.
This limit is not to be understood as a deficiency, but as a clarification. It separates the legitimate achievements of materialist explanations from metaphysical expectations they cannot fulfill. In this sense, the present investigation does not contribute to the abolition of materialist conceptions of the self, but to their conceptual discipline.
The framework is thereby delineated within which further ontological models can be meaningfully discussed. The question of the self does not turn out to be an empirical riddle that could be solved by better data, but an epistemological threshold at which it is decided what kind of explanation one expects at all.
Anyone who maintains ontological materialism while simultaneously rejecting metaphysical supplementary assumptions thereby accepts a principled fragility of personal identity. This fragility is not a flaw of the model, but a legitimate consequence of its ontological parsimony when applied to the self.
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The present paper examines the limits of ontological–materialist explanations of personal identity under an exclusivity claim. Throughout, the argument deliberately operates within an objectifying descriptive framework. As a complement, however, it is useful to make explicit an epistemic precondition of this framework in order to avoid potential misunderstandings.
Every materialist self-description presupposes a perspective from which this description is carried out. Within ontological materialism, the self appears as an object within a physically described world, for example as a functionally realized system or as a biological organism. This objectification is methodologically legitimate and indispensable for scientific explanation. At the same time, it already presupposes the existence of an experiencing subject for whom the world appears as describable.
Experience as experience is not objectifiable in the same way as its neurophysiological correlates. It is not itself an object of external measurement or observation, but rather constitutes the epistemic condition under which observation, description, and identity attribution are possible at all. Materialist models therefore do not capture experience as such, but the objectifiable structures and processes that, within a stipulated world description, correlate with experience.
The fragility of personal identity diagnosed in the main text therefore does not concern experience itself, but the ontological projection in which the self is to be fixed as an exclusively identifiable object within a materialist framework. The analysis thus marks a limit of ontologization, not a denial of the reality of subjective experience.
This epistemic remark supplements the main argument but is not required for its validity. The central result remains an internal consistency analysis of materialist identity claims and stands independently of further epistemological or metaphysical commitments.