Yes — in QBism the word belief has a technical, non-colloquial meaning, though it deliberately draws on the everyday sense too.

Here’s the breakdown:

• Colloquially, "belief" often means holding something to be true about the world (e.g., “I believe it will rain tomorrow”).

• In QBism, belief is used in the Bayesian / decision-theoretic sense:

  • A belief is an agent’s personal, subjective degree of expectation about the outcome of an action they might take on the world.

  • These are represented numerically as probabilities in the Bayesian sense — not as objective chances “out there,” but as the agent’s own commitments about what they expect.

  • To say “I believe with probability 0.7 that I’ll see outcome X if I perform measurement Y” is to make a quantified commitment about expectations, not to state a fact about the external world.

• This connects to a core QBist move: the quantum state (the wavefunction) itself is not a description of the world but a mathematical expression of an agent’s beliefs about their own experiences.

• Importantly, in QBism, a belief is not just a vague feeling — it is something action-guiding and accountable. If your probability assignments don’t cohere (violating Dutch-book consistency, for example), you risk being "irrational" in the Bayesian sense.

So:

• In everyday speech: belief ≈ “thinking something is true.”

• In QBism: belief = “an agent’s personal, Bayesian degree of expectation, expressed in probabilities, about the consequences of their actions on the world.”

It’s technical because it explicitly rejects the idea that probabilities describe objective features of reality. Instead, belief is what each agent brings into play when engaging with the world.

Would you like me to contrast that with how "knowledge" is treated in QBism? (Because Fuchs & Schack are careful to reject the old idea of a "quantum state as knowledge," insisting it’s strictly belief/expectation instead.)

You said:

ok

ChatGPT said:

In QBism, “belief” and “knowledge” are deliberately pulled apart:

• Knowledge usually implies something objective and true about the world (“I know the mass of the electron is such-and-such”). In physics, this carries the sense that knowledge is impersonal and verifiable by anyone.

• Belief in QBism, by contrast, is personal and agent-specific. A quantum state is not knowledge about the world “as it is,” but a bookkeeping device for the agent’s own expectations about the consequences of their actions.

Chris Fuchs, Rüdiger Schack, and others emphasize that calling the quantum state “knowledge” (as some interpretations did, e.g. quantum Bayesianism in its early form) is misleading because it makes people think the wavefunction encodes an objective feature of reality. That’s why they shifted to the stronger word belief, to stress its subjective Bayesian grounding:

• A wavefunction encodes nothing more and nothing less than the agent’s betting commitments.

• When measurement outcomes occur, they update beliefs (via Bayes’ rule) — not “gain knowledge” of a hidden property.

So, in short:

• Knowledge = impersonal, objective, fact-like (QBism avoids this word for states).

• Belief = personal, probabilistic expectation guiding action, the technical sense QBism uses.

Do you want me to also unpack how this subjective belief framework still manages to give rise to shared, intersubjective science — i.e. how QBism explains why physics isn’t just solipsism?

ChatGPT said:

Great question — and one that goes right to the heart of what QBism is trying to do.

The short answer: No, quantum mechanics does not “become classical” in QBism. But the relationship to classical physics shifts in an interesting way.

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1. What “classical” usually means

• In the usual story, classical physics is about objective states of the world (e.g., positions and velocities), while quantum mechanics introduces superposition, indeterminacy, collapse, etc.

• Some interpretations say classical behavior emerges from quantum physics at large scales (e.g., via decoherence).

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2. QBism’s twist

QBism reframes both classical and quantum physics under the same Bayesian umbrella:

• Probabilities everywhere:

  • In classical statistical mechanics, probabilities are often treated as “ignorance” about some hidden state.

  • In QBism, probabilities — whether in a coin toss, in thermodynamics, or in quantum theory — are always personal beliefs about outcomes, never objective facts about hidden states.

• Quantum vs. classical difference:

  • The difference is not that quantum mechanics is more “mysterious” or more “objective.”

  • The difference is that quantum mechanics places stricter consistency constraints on an agent’s probability assignments than classical probability theory does.

  • These constraints (expressed in the Born rule) are what make QM special — not the existence of a new kind of reality behind the probabilities.

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3. Larger scales and the classical limit

• At large scales, when quantum interference is negligible, the QBist formalism does indeed approximate classical probabilistic reasoning.

• But the conceptual stance doesn’t change: even in the classical case, probabilities = personal beliefs.

• What happens is that the “quantum bookkeeping rules” become less noticeable, so things look more like standard classical probability theory.

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4. Big picture

So in QBism:

• Quantum mechanics does not reduce to classical mechanics.

• Instead, classical mechanics is just a special case of probability assignments where the quantum constraints become irrelevant.

• The real move is: both quantum and classical physics are understood as user manuals for agents making bets about experiences.

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👉 Put differently: In Copenhagen, quantum → classical via decoherence or measurement.
In QBism, both are “belief calculi,” but quantum is a richer one.

Do you want me to sketch how Fuchs frames this in terms of the Born rule as a normative addition to probability theory — i.e. why QM is “more than Bayesian but not objective”?

You said:

Yes

ChatGPT said:

Alright, let’s go step by step into how Fuchs (and Schack) cast the Born rule in QBism as a normative addition to standard Bayesian probability:

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1. Bayesian baseline

In Bayesian probability theory:

• Probabilities are an agent’s personal degrees of belief.

• Rationality is enforced by coherence: your probability assignments shouldn’t allow a “Dutch book” (a guaranteed loss).

• These rules (like the sum rule and Bayes’ theorem) are internal consistency conditions, nothing about “the world itself.”

So Bayesianism gives you a clean self-consistency check — but it doesn’t tell you anything about how your beliefs should relate to the physical world you interact with.

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2. Where quantum mechanics comes in

In QBism, quantum mechanics enters here:

• The Born rule is not a statement about hidden variables or objective chances.

• Instead, it is a new, empirical, normative rule telling agents how their personal probability assignments must hang together if they want to use the world successfully.

So:

• Probability theory gives internal consistency.

• Quantum mechanics adds external normative guidance.

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3. Born rule as a consistency constraint

Fuchs often frames it like this:

The Born rule is not about predicting “what is.” It’s about how to adjust your beliefs about one set of potential actions given your beliefs about another.

For example, using a symmetric informationally complete POVM (SIC-POVM), the Born rule can be rewritten as:

$$Q(D_j) = \sum_i \left[(d+1) P(H_i) - \frac{1}{d}\right] P(D_j|H_i)$$

Here:

• $P(H_i)$ = the agent’s personal probabilities for outcomes of one “fiducial” measurement.

• $P(D_j|H_i)$ = conditional probabilities for outcomes of another measurement.

• $Q(D_j)$ = the agent’s overall probability for outcome $D_j$.

This relation shows the Born rule as a constraint connecting different belief assignments. It doesn’t describe objective reality; it tells you how to keep your probability web in tune with the world you’re embedded in.

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4. Why this is more than “classical”

• Classical Bayesian probability doesn’t impose this extra rule.

• In that sense, QM is Bayesian probability + an empirical supplement (the Born rule).

• It’s like saying: “Here are the basic rules of rational betting. But if you live in a quantum world, there’s an extra structure you must respect.”

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5. Fuchs’ punchline

That’s why Fuchs sometimes says:

• Quantum theory is not a description of the world.

• It’s a user’s manual for agents, where the Born rule is a special instruction about how to manage expectations in this particular world.

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So to your original question:

• No, QBism doesn’t make QM collapse into classical physics.

• Instead, it makes classical physics look like a looser, less-constrained version of probability theory, while QM is the tighter, richer version thanks to the Born rule’s normative addition.

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Would you like me to also lay out how this Born-rule-as-norm idea is used by QBists to argue that QM is “universal” — i.e., not just about microphysics but equally about macroscopic systems, even living beings?

You said:

If you remove agents, how does the universe/reality look form a QBist perspective?

ChatGPT said:

That’s the sharp edge of QBism — because if you “remove agents,” you’ve pulled away the very thing QBism is built around. Let me walk carefully:

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1. No agent, no probabilities

• In QBism, quantum states, probabilities, the Born rule — all of that is about an agent’s expectations regarding their experiences.

• If there is no agent, then there are no probabilities, no quantum states, no wavefunctions “out there.”

• These are tools for an agent, not ontological furniture.

So from a QBist perspective: you don’t get a picture of the universe at all if you strip away agents. The formalism is silent.

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2. But QBism doesn’t collapse into solipsism

Fuchs is careful to insist:

• Agents are in the world, not floating brains.

• An agent is always interacting with something beyond them — QBism takes that “something beyond” very seriously.

• The world pushes back, delivers surprises, and resists full control.

So while QM is about beliefs, it’s not “all in your head.” It’s about how your beliefs should be structured given that there’s a world out there that answers you in unpredictable ways.

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3. The “world without agents” problem

If you insist on imagining the universe with no agents, QBism has to say something like:

• There would still be a reality, but QBism makes no claims about its character apart from:

  • It is not fully capturable in mathematical description.

  • It is the source of surprises.

  • It is what agents bump up against when they act.

So the world exists, but quantum mechanics is not its portrait. Without agents, the user’s manual has no user.

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4. Fuchs’ language for this

Fuchs sometimes says:

• Quantum mechanics doesn’t describe reality; it’s a “user’s manual for living in reality.”

• Remove the user, and you have reality still there — but quantum theory falls silent.

In that sense:

• Physics without agents = there is a world, but no probabilities, no wavefunctions, no quantum states.

• QBism neither denies the world nor paints it — it only tells you how to navigate it as an agent.

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5. Big contrast

• In Everett (Many-Worlds): remove agents → you still have the universal wavefunction evolving deterministically.

• In QBism: remove agents → the wavefunction itself disappears (since it was never “out there” to begin with). All that remains is a mute, resistant, never-fully-known reality.

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👉 So the cleanest way to say it:
From a QBist perspective, reality without agents is ineffable. Quantum theory cannot picture it — it only guides agents within it.

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Do you want me to sketch how Fuchs tries to connect this ineffable “something beyond” to a kind of philosophical realism — i.e. why he thinks QBism is not just idealism dressed up?

You said:

Yes

ChatGPT said:

Alright — this is exactly the nerve QBism is often poked at (“isn’t this just solipsism or idealism in disguise?”). Fuchs spends a lot of ink trying to show that it isn’t.

Here’s how he frames a kind of QBist realism:

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1. Agents are real, but so is what they act on

• An agent doesn’t invent the world.

• The world is what resists, surprises, and pushes back when the agent acts.

• Quantum mechanics does not describe this world directly; instead, it encodes the norms an agent must follow when making bets in that world.

So the world has a stubborn, independent reality. What is subjective is the agent’s probability assignments, not the world’s existence.

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2. The ineffable “something more”

Fuchs often invokes William James and pragmatism:

• The world is “more than my account of it.”

• Quantum theory is a map for navigating, but the terrain is inexhaustible.

• This is why quantum outcomes are genuinely unpredictable: each measurement brings something new into the world.

So reality is not just “hidden variables we haven’t found yet.” It’s fundamentally more than any description, and that’s what grounds QBism’s realism.

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3. Quantum theory as a user’s manual

• Think of a GPS: it tells you how to navigate, but it isn’t the landscape itself.

• The map presupposes the landscape exists — otherwise, the directions would be meaningless.

• Likewise, quantum theory presupposes a real world that pushes back when agents act.

So, even though QBism denies that the wavefunction is a description of reality, it insists that the need for such a manual testifies to reality’s independence.

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4. Realism without representation

Fuchs’ position is sometimes called a nonrepresentational realism:

• Quantum theory doesn’t represent the world.

• But the very fact that agents need normative rules to coordinate their expectations means the world isn’t arbitrary or agent-created.

• The Born rule, as an empirical constraint, is the “voice of the world” in the agent’s reasoning — not as a picture, but as a normative tug.

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5. Fuchs’ metaphysical hunch

He sometimes goes further, speculating that:

• Each quantum event (each “measurement outcome”) is an act of creation, where something genuinely new comes into being.

• Reality is not a fixed block but an ever-growing, pluralistic unfolding — consistent with James’ “pluriverse” rather than a monistic universe.

This is where QBism shades into metaphysics: it’s not just epistemology, but a way of saying that unpredictability itself is the most direct sign of reality’s independence.

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👉 So the punchline:

• QBism rejects “realism = description.”

• But it affirms a pragmatist realism: there really is a world, it is more than agents, and its resistance is what makes probability assignments non-arbitrary.

• Agents bring beliefs, the world brings surprises, and quantum theory is the bridge.

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Do you want me to sketch the philosophical parallels Fuchs explicitly draws — e.g. to William James, John Dewey, and pragmatist “radical empiricism” — that give QBist realism its shape?