One World, Many Perspectives: Relational Quantum Mechanics, QBism, Leibniz, and Taoism

Relational quantum mechanics and QBism both reject the idea that quantum theory offers a view from nowhere. Yet they understand the observer, reality, and the relation between perspectives in importantly different ways. Leibniz and Taoism can illuminate this difference, provided that philosophical comparison is not mistaken for physical evidence. Leibniz’s harmony also raises a narrower theological question about responsibility.

The question behind the interpretations

A discussion about personal identity and moral responsibility recently led me back to a more general question:

If reality is always encountered from a perspective, what makes different perspectives perspectives on the same world?

This question appears in several forms. In special relativity, different observers assign different coordinates to the same events. In quantum mechanics, different observers may assign different states—or even different sets of definite facts—to the same physical system. In philosophy of mind, several moments of experience are nevertheless attributed to one persisting person. And in ethics, actions performed in the past may still be attributed to the person who exists today.

These are not versions of the same technical problem. Quantum theory cannot by itself determine what personal identity or moral responsibility means. But they share a structural question: how can plurality remain coherent without being reduced to a single privileged perspective?

Two forms of quantum perspectivalism

Relational quantum mechanics, or RQM, was introduced by Carlo Rovelli. Its central claim is that the quantum state and the values of physical variables are not absolute properties of a system. They are relative to another physical system.

An ‘observer’ in RQM need not be conscious. It can be a measuring apparatus, an atom, or any other physical system. A quantum fact is realised in an interaction between systems. In the scenario known as Wigner’s friend, an outcome can be definite relative to a laboratory observer while an external observer still describes the laboratory and measured system as entangled.

RQM therefore does not divide the universe into a quantum domain and a special classical or mental domain. All physical systems are treated on the same footing. It describes one interacting physical world, but not one global, observer-independent catalogue of every fact. Rovelli’s claim is precisely that the network of relational descriptions can be complete without being replaced by a final description from nowhere.

QBism—originally ‘Quantum Bayesianism’—is perspectival in a different sense. Here the central figure is not merely a physical reference system but an agent: something that chooses actions and experiences their consequences.

For a QBist, a quantum state expresses an agent’s personal probabilities for the possible consequences of actions upon the external world. The Born rule is therefore normative rather than mechanical. It does not describe a physical mechanism pushing the world from one state to another. It constrains how an agent’s probability assignments should fit together.

The word normative has a precise meaning here. It does not simply mean that quantum mechanics is a symbolic aid for interpreting life. It means that the formalism supplies rational constraints on an agent’s expectations and decisions.

QBism is not solipsism

It is tempting to describe QBism as solipsistic because every application of the theory begins from the standpoint of a particular agent. But this would be inaccurate.

QBism explicitly assumes a world external to the agent. A measurement is an action upon that world, and the result is not under the agent’s control. The world can surprise the agent. Other agents also belong to this external world, and their reports can enter my experience through communication.

QBism is thus a ‘single-user’ theory, but this does not mean that only one user exists. It means that every concrete probability assignment belongs to some particular agent. Different agents can communicate and construct a common body of reality.

There is nevertheless a genuine question here. According to Rüdiger Schack, nothing in the QBist formalism automatically requires the quantum-state assignments or measurement outcomes of two agents to be mutually consistent. Agents can create conditions under which they expect agreement, but intersubjective agreement is a goal rather than an automatic theorem of the formalism.

The strongest objection is therefore not that QBism places each person in a separate universe. It is more precise:

QBism assumes an external world and the possibility of communication, but its quantum-mechanical norm is intra-agentic. The formalism itself does not provide a general principle guaranteeing coherence between different agents’ perspectives.

Does RQM solve the problem?

RQM appears to offer a more directly physical account of relations between perspectives. Observers are physical systems, information is physically stored, and the comparison of two observations is itself a physical interaction.

This also answers a common regress objection. Suppose an apparatus measures a quantum system. Because the apparatus can itself be described quantum mechanically, an external observer may describe the apparatus and system together. That external observer can then be included in a still larger description, and so on.

But RQM does not require this sequence to be completed before the first event becomes real. The event is already a fact relative to the systems that interacted. A further observer is needed only if we want a new description from a further perspective.

The regress becomes vicious only if we demand a final observer who provides an absolute description of every other observer while somehow also including its own perspective. RQM rejects that demand from the outset.

A subtler problem remains. If Alice has recorded an outcome, under what conditions must Bob later obtain a matching result when he reads Alice’s record?

Before cross-perspective links were proposed, RQM already contained a weaker form of cross-perspectival coherence. Di Biagio and Rovelli call a relative fact stable for a further system when it can be used in the classical law of total probability for that system’s possible future facts: probabilities then combine as a classical mixture rather than through interfering amplitudes. Decoherence is the usual physical mechanism, entangling a record with environmental degrees of freedom that are inaccessible to the further system and thereby suppressing the relevant interference terms in its probabilities.

The qualification matters. Stability is approximate and itself relational: a differently coupled system may still detect interference. A stable fact is therefore neither absolute nor necessarily already observed by the system for which it is stable.

Emily Adlam and Carlo Rovelli add a stronger condition with their postulate of cross-perspective links, or CPL. If information about Alice’s result remains physically stored and has not been destroyed, Bob must recover the corresponding result when he later measures that record appropriately.

CPL can therefore be read as an embryo of a cross-perspectival consistency rule. It gives certain events a degree of observer-independent accessibility while quantum states remain relational. But it is not yet a general transformation law mapping Alice’s complete description into Bob’s, and its place within RQM remains a matter of debate.

The experimental edge

The issue is not merely verbal. Extended Wigner’s-friend arguments turn assumptions about shared facts into experimentally constrained propositions. Časlav Brukner showed that universal quantum validity, locality, freedom of choice, and observer-independent facts (OIF) cannot all be retained. Bong and collaborators later showed—provided that coherent quantum control extends to the scale of an observer—that no-superdeterminism, locality, and the absoluteness of observed events (AOE) are jointly incompatible with the quantum correlations predicted in their scenario.

The experimental demonstration by Bong and collaborators was explicitly a photonic proof of principle: a two-path photonic qubit stood in for each ‘friend’, not a macroscopic or sentient observer. The result therefore does not establish that human experiences are relative facts. It tests the relevant inequality in a deliberately minimal physical implementation.

These no-go results do not by themselves select RQM, QBism, or any other interpretation. They show that specific formal notions of observer-independent facts—Brukner’s OIF and Bong et al.’s weaker AOE—cannot be combined with their respective remaining assumptions and the relevant quantum predictions. I discussed this experimental pressure in more detail in Newton or Leibniz? Modern Physics and Two Concepts of Reality.

The lesson from relativity—and its limit

RQM is partly inspired by relativity. Velocity is not an absolute property of an object; it is velocity relative to a reference frame. Special relativity nevertheless does more than declare all coordinates relative. It supplies Lorentz transformations connecting coordinate systems and identifies invariant structure, such as the spacetime interval.

This is what prevents relativity from dissolving into disconnected descriptions. Different observers do not merely possess private accounts. Their descriptions are related by precise transformation laws.

Standard RQM does not yet offer an equally general transformation law translating the complete set of facts relative to one system into the complete set relative to another. Comparison is itself an interaction, and the interaction may change or destroy some of the information being compared. CPL is an important fragment: it supplies a conditional matching rule for a preserved physical record. But it does not provide the analogue of a Lorentz transformation between complete descriptions.

This motivates a broader principle that I call observer-equivariance, developed more formally in this preprint:

Objectivity is not the absence of perspective. It is what remains invariant, or transforms coherently, under admissible changes of perspective.

This principle is stronger than merely saying that all descriptions are relative. It asks:

Which parts of a description depend on perspective?
Which changes of perspective are admissible?
How is information transported when the perspective changes?
Which laws or structures remain invariant under that transport?
When does local information combine into a shared global object?
When is such a combination mathematically or physically obstructed?

Observer-equivariance should not be identified with RQM. It is better understood as a general objectivity requirement that can be used to analyse, criticise, or extend both RQM and QBism. RQM supplies a fragment of cross-perspectival consistency; observer-equivariance asks after the larger structure.

Leibniz: many perspectives on one universe

Gottfried Wilhelm Leibniz provides an older and more explicitly metaphysical model of perspectival reality. Each monad expresses the entire universe from its own point of view. The plurality of perspectives does not divide reality into separate universes; each perspective belongs to the same cosmic order.

This resembles the idea that objectivity might be found between perspectives rather than outside all perspective. But the analogy has strict limits.

Leibniz’s monads are ‘windowless’. Created monads do not causally interact. Their states correspond because God has established a harmony between their internally generated histories. RQM is almost the opposite in this respect: relational quantum facts arise precisely through physical interaction.

Nor is Leibniz’s God best understood as an editor who continually adjusts already independent decisions. The harmony is pre-established. God creates a complete world in which each monad develops according to its own internal principle while remaining coordinated with all the others.

A structural relation between perspectives does not, however, require a conscious cosmic editor. Compatibility may be immanent in the laws themselves. Lorentz covariance does not need an entity that continually synchronises reference frames. In the same way, a future theory of quantum perspectives might contain consistency conditions without turning those conditions into a divine agent.

Harmony, freedom, and divine responsibility

Leibniz’s pre-established harmony raises a theological problem. If God knowingly selects and sustains the entire order of the world, including every human history, can responsibility for evil be assigned entirely to created beings? Leibniz answers that God permits moral evil but does not will it as evil: imperfection belongs to finite creatures, while God chooses the best world available among the possible worlds. Whether this successfully removes divine responsibility is one of the central difficulties of his theodicy.

One might instead argue that a God who sustains the whole must in some sense also assume responsibility for the whole. This can be connected with Christian ideas of atonement: God does not merely judge the consequences of the created order from outside but enters that order and bears its suffering. Yet to bear the consequences of sin is not the same as being morally guilty of committing it.

None of this follows from RQM, observer-equivariance, or the mathematics of relational consistency. It depends on additional premises about creation, freedom, providence, and incarnation. The direction of inference therefore matters: this is a question opened by the metaphysics, not a result derived from quantum theory. I discuss that distinction more generally in When Science Meets Spirituality: Dialogue, Synthesis, and the Direction of Inference.

Taoism: process without a cosmic editor

Taoism, or Daoism, offers a different philosophical contrast. Classical Taoist texts such as the Dao De Jing and the Zhuangzi repeatedly resist the attempt to understand reality as a collection of completely fixed, self-sufficient things. They emphasise transformation, dependence, changing perspectives, and ways of acting that do not impose an artificial order upon natural processes.

Chapter 2 of the Zhuangzi, the Qiwulun, is especially relevant. Its shifts between ‘this’ and ‘that’, and its refusal to appoint a final judge standing outside every perspective, do not amount to the claim that every view is equally adequate. They challenge the assumption that disagreement can always be settled from an unconditioned standpoint.

This must not be turned into the claim that ancient Taoism anticipated quantum mechanics. Similarities in vocabulary are not physical predictions, and quantum experiments do not verify Taoist metaphysics.

Taoism can nevertheless clarify a philosophical possibility. Coherence need not be imposed by an external controller. Order can be immanent in the way processes unfold. The Dao is not a divine engineer standing outside the world and continually correcting its course. Taoist notions such as ziran—often rendered as naturalness or ‘self-so-ness’—suggest an order arising through the character of the process itself.

The Taoist idea of wu wei is also frequently misunderstood. It does not simply mean passivity or literal inaction. It concerns action that does not force events into an externally imposed pattern. In the present context, it can serve as a philosophical counterweight to the image of the universe as a centrally managed machine.

Leibniz and Taoism can therefore be placed on opposite sides of a useful contrast:

Leibniz explains perspectival harmony through a divinely pre-established total order.
Taoist thought allows us to imagine coherence as immanent in ongoing transformation, without a separate cosmic coordinator.

Neither alternative follows from quantum theory. They are metaphysical interpretations of what coherence might mean.

Flow and structure are not opposites

The contrast between a static ontology and a dynamic flow can itself be misleading. A structure need not be frozen, and a flow is not intelligible without some persistence of relations.

Physics is full of structures that govern change: dynamical symmetries, conservation laws, causal relations, transition amplitudes, and transformation groups. Describing these structures does not require standing outside the universe or treating it as a completed dead object. A law can be invariant while the states governed by the law continually change.

Observer-equivariance is therefore compatible with a process-oriented ontology. Its purpose is not to replace becoming with a static master description. It asks which patterns make communication, comparison, and shared science possible within the process.

This is where Taoist and structural language can meet without being confused. Taoism reminds us not to mistake abstract objects for self-sufficient substances. Mathematics reminds us that an immanent flow can still possess exact relational form.

The observer is not yet a self

The words observer and agent can encourage an unwarranted leap from quantum foundations to theories of the human self.

In RQM, an observer may be an elementary physical system. It therefore has no necessary memory, consciousness, intention, or moral responsibility. QBism gives the agent a more central role, but it still does not provide a theory of personal identity across time.

Questions of guilt and responsibility require further concepts: continuity of the person, knowledge of the consequences, control over the action, intention, coercion, memory, and the distinction between moral blame and the social purposes of punishment. None of these can be read directly from the Born rule.

Quantum interpretations may alter our general image of agency, determinism, and the openness of the future. They do not decide who is responsible for a past action.

Objectivity between perspectives

The difference between RQM and QBism should therefore not be reduced to ‘one shared reality versus solipsism’.

RQM describes a world of physical systems in which facts arise relationally through interaction. It rejects a global quantum state while attempting to preserve a realist account of physical events.

QBism treats quantum theory as a normative calculus used by an agent acting upon an external world. It is not solipsistic, but the formalism is deliberately centred on one agent at a time, and intersubjective agreement is not automatically imposed by it.

Leibniz shows how many perspectives might express one universe through a pre-established harmony. Taoism offers a contrasting image of order as immanent in transformation rather than imposed by a cosmic editor. Both can sharpen the philosophical question, but neither should be presented as an interpretation experimentally derived from quantum mechanics.

The central problem remains:

What structure allows different perspectives to be genuinely different while still belonging to one scientifically intelligible world?

Perhaps objectivity is not found above all perspectives, in an impossible view from nowhere. Perhaps it is found in the disciplined passage between them.

Related work

References and further reading

Carlo Rovelli, “Relational Quantum Mechanics” (1996): arXiv:quant-ph/9609002.
Carlo Rovelli, “The Relational Interpretation of Quantum Physics” (2021): arXiv:2109.09170.
Andrea Di Biagio and Carlo Rovelli, “Stable Facts, Relative Facts” (2021): doi:10.1007/s10701-021-00429-w.
Emily Adlam and Carlo Rovelli, “Information is Physical: Cross-Perspective Links in Relational Quantum Mechanics” (2023): doi:10.31389/pop.8.
Christopher A. Fuchs, N. David Mermin, and Rüdiger Schack, “An Introduction to QBism with an Application to the Locality of Quantum Mechanics” (2014): arXiv:1311.5253.
Christopher A. Fuchs, “QBism, Where Next?” (2023): arXiv:2303.01446.
Rüdiger Schack, “When Will Two Agents Agree on a Quantum Measurement Outcome?” (2023): arXiv:2312.07728.
Časlav Brukner, “A No-Go Theorem for Observer-Independent Facts” (2018): doi:10.3390/e20050350.
Kok-Wei Bong et al., “A Strong No-Go Theorem on the Wigner’s Friend Paradox” (2020): doi:10.1038/s41567-020-0990-x.
Gottfried Wilhelm Leibniz, Monadology: online text.
Gottfried Wilhelm Leibniz, Theodicy: Project Gutenberg.
Dao De Jing: Chinese Text Project.
Zhuangzi, Chapter 2, Qiwulun: Chinese Text Project.
“Daoism”, Stanford Encyclopedia of Philosophy: plato.stanford.edu.

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Gustaf Ullman — Perspectives on Physics and Philosophy