On QBism and Assumption (Q)

Blake C. Stacey

arxiv: 1907.03805 · v2 · pith:QKRKHTZBnew · submitted 2019-07-08 · 🪐 quant-ph · physics.hist-ph

On QBism and Assumption (Q)

Blake C. Stacey This is my paper

Pith reviewed 2026-05-25 00:53 UTC · model grok-4.3

classification 🪐 quant-ph physics.hist-ph

keywords QBismWigner's Friendquantum foundationsthought experimentsquantum interpretationsmeasurement

0 comments

The pith

Recent papers on Wigner's Friend extensions misrepresent QBist beliefs both historically and conceptually.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper identifies and corrects two misapprehensions in the recent literature extending the Wigner's Friend thought-experiment. One error is historical and the other conceptual; both involve inaccurate descriptions of some quantum physicists' beliefs by others. If the corrections are right, then analyses of observer-dependent quantum states rest on mistaken attributions of what QBism actually holds. A reader would care because debates over quantum measurement require precise accounts of the positions under discussion.

Core claim

The author establishes that the recent literature on Wigner's Friend extensions contains a historical misapprehension and a conceptual misapprehension, both of which concern the accurate description of quantum physicists' beliefs, particularly those of QBists, by other authors.

What carries the argument

Assumption (Q), the conceptual premise whose attribution or relation to QBism is corrected.

If this is right

Future work on Wigner's Friend scenarios must employ the actual QBist position rather than the misdescribed version.
Historical accounts of quantum interpretations require revision to reflect the corrected description of QBism.
Conceptual arguments about quantum states as agent beliefs would shift once the misattributions are removed.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

Similar descriptive inaccuracies may appear in treatments of other quantum interpretations in thought-experiment literature.
Direct self-descriptions by the physicists whose views are summarized could become standard in foundational papers.
Clarification here opens the possibility of constructing new extensions of Wigner's Friend that properly incorporate the QBist stance.

Load-bearing premise

The author's reading of the recent literature accurately identifies the claimed misapprehensions and correctly represents the beliefs of the physicists in question.

What would settle it

Direct confirmation from the QBist physicists in question stating whether their beliefs align with the author's corrections or with the descriptions given in the critiqued papers.

read the original abstract

I correct two misapprehensions, one historical and one conceptual, in the recent literature on extensions of the Wigner's Friend thought-experiment. Perhaps fittingly, both concern the accurate description of some quantum physicists' beliefs by others.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

Stacey's short note flags two specific misrepresentations of QBist positions in recent Wigner's friend papers, but the value hinges on whether those readings are accurate.

read the letter

This paper is a targeted correction note. It identifies one historical and one conceptual misapprehension in how recent extensions of the Wigner's friend scenario have described QBist beliefs, especially around assumption (Q). The main service is reminding readers that attributions of positions need to match what the physicists in question actually hold. Stacey keeps the scope narrow and sticks to that point without claiming new derivations or experimental implications. That directness is useful in a subfield where descriptions can drift. The piece does not introduce new math, data, or predictions, so there are no formal or empirical claims to check for soundness. The citation pattern is limited to the papers being critiqued, which is appropriate for the task. The central limitation is that the corrections depend entirely on the author's interpretation of the source material being the correct one. If the original papers meant something different or if the beliefs are stated with more nuance than presented here, the identified misapprehensions do not hold. The note itself provides little in the way of extended quotes or side-by-side comparisons, so verification requires going back to the cited works. This is typical for commentary pieces but means the strength is not independently verifiable from the text alone. The work is only relevant to readers already following the QBism literature and the specific Wigner's friend papers. Outsiders or those looking for new results will find little to engage with. It does not rise to the level that would justify sending it for full peer review; a short comment or arXiv posting is the right format if the points need to be on record.

Referee Report

2 major / 0 minor

Summary. The manuscript claims to correct two misapprehensions—one historical and one conceptual—in the recent literature on extensions of the Wigner's Friend thought-experiment. Both misapprehensions concern the accurate description of some quantum physicists' beliefs by others, specifically in relation to QBism and Assumption (Q).

Significance. If the author's interpretations of the cited literature prove accurate, the note would contribute to clearer scholarly communication in quantum foundations by reducing the risk of propagating inaccurate characterizations of positions such as QBism. Its significance is primarily clarificatory rather than advancing new formal results, predictions, or empirical tests.

major comments (2)

[Abstract] The central claim that two specific misapprehensions exist in the literature is load-bearing and rests entirely on the accuracy of the author's reading of others' work. The manuscript provides no direct quotations, page references, or detailed summaries of the statements being corrected, preventing independent verification of the mismatch between the literature's claims and the physicists' actual beliefs.
[Abstract] The weakest assumption identified in the stress-test note—that the author's reading accurately captures the literature—is not addressed with evidence in the text. Without such substantiation, the corrections cannot be evaluated as sound or proportionate to the claimed errors.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their report and the opportunity to clarify our manuscript. We address each major comment below and indicate revisions to improve verifiability of the claims.

read point-by-point responses

Referee: [Abstract] The central claim that two specific misapprehensions exist in the literature is load-bearing and rests entirely on the accuracy of the author's reading of others' work. The manuscript provides no direct quotations, page references, or detailed summaries of the statements being corrected, preventing independent verification of the mismatch between the literature's claims and the physicists' actual beliefs.

Authors: We agree that direct quotations, page references, and brief summaries of the targeted statements would facilitate independent verification. The revised manuscript will incorporate specific excerpts from the cited works (e.g., the relevant passages on QBism and Assumption (Q) in the Wigner's Friend literature) to make the identified mismatches explicit and checkable. revision: yes
Referee: [Abstract] The weakest assumption identified in the stress-test note—that the author's reading accurately captures the literature—is not addressed with evidence in the text. Without such substantiation, the corrections cannot be evaluated as sound or proportionate to the claimed errors.

Authors: The note's core contribution is the identification of the two misapprehensions. To meet the referee's concern, the revision will supply the requested textual evidence (quotations and references) so that readers can assess whether our characterizations are accurate and the corrections proportionate. No new formal results are claimed; the paper remains clarificatory. revision: yes

Circularity Check

0 steps flagged

No circularity; paper is external interpretive commentary without derivation chain.

full rationale

The paper is a short note whose central claim is the existence of two specific misapprehensions (historical and conceptual) in the recent Wigner's Friend literature concerning accurate descriptions of QBist beliefs and Assumption (Q). No equations, parameters, predictions, or first-principles derivations appear. The enumerated circularity patterns (self-definitional, fitted-input-as-prediction, self-citation load-bearing, uniqueness imported from authors, ansatz smuggled via citation, renaming known result) do not apply because there is no derivation to reduce to its inputs. The accuracy of the author's reading of cited sources is a correctness issue, not a circularity issue within any claimed derivation. The paper is therefore self-contained as commentary.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Based on abstract only, the paper rests on domain assumptions about the content of QBism and the Wigner's Friend literature without introducing new parameters or entities.

axioms (1)

domain assumption Standard domain assumptions about the content and history of QBism and related quantum interpretations.
The corrections presuppose familiarity with what QBism and Assumption (Q) entail in the cited literature.

pith-pipeline@v0.9.0 · 5539 in / 953 out tokens · 21594 ms · 2026-05-25T00:53:22.709027+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

17 extracted references · 17 canonical work pages · 11 internal anchors

[1]

The hidden assumptions of Frauchiger and Renner,

A. Sudbery, “The hidden assumptions of Frauchiger and Renner,”arXiv:1905.13248 (2019)

work page arXiv 1905
[2]

Quantum theory cannot consistently describe the use of itself

D. Frauchiger and R. Renner, “Quantum theory cannot consistently describe the use of itself,” Nature Communications 9 (2018), 3711, arXiv:1604.07422

work page internal anchor Pith review Pith/arXiv arXiv 2018
[3]

Remarks on the mind-body question

E. P. Wigner, “Remarks on the mind-body question.” InThe Scientist Speculates, edited by I. J. Good (Heinemann, 1961)

work page 1961
[4]

Quantum probabilities as Bayesian probabilities

C. M. Caves, C. A. Fuchs and R. Schack, “Quantum probabilities as Bayesian probabilities,” Physical Review A65 (2002), 022305, arXiv:quant-ph/0106133

work page internal anchor Pith review Pith/arXiv arXiv 2002
[5]

C. A. Fuchs, numerous conversations (2012–19); C. M. Caves and R. Schack, e-mail (2019)

work page 2012
[6]

Subjective probability and quantum certainty

C. M. Caves, C. A. Fuchs and R. Schack, “Subjective probability and quantum certainty,” Studies in the History and Philosophy of Modern Physics 38 (2007), 255–74, arXiv: quant-ph/0608190

work page internal anchor Pith review Pith/arXiv arXiv 2007
[7]

Quantum-Bayesian Coherence

C. A. Fuchs and R. Schack, “Quantum-Bayesian Coherence”arXiv:0906.2187 (2009)

work page internal anchor Pith review Pith/arXiv arXiv 2009
[8]

Two dogmas about quantum mechanics

J. Bub and I. Pitowsky, “Two dogmas about quantum mechanics.” InMany Worlds?: Everett, Quantum Theory, & Reality(Oxford University Press, 2010),arXiv:0712.4258

work page internal anchor Pith review Pith/arXiv arXiv 2010
[9]

Bayesian Probability Theory and Quantum Mechanics,

J. C. Baez, “Bayesian Probability Theory and Quantum Mechanics,”http://math.ucr.edu/ home/baez/bayes.html (2003)

work page 2003
[10]

H. C. von Baeyer,QBism: The Future of Quantum Physics(Harvard University Press, 2016)

work page 2016
[11]

An Introduction to QBism with an Application to the Locality of Quantum Mechanics

C. A. Fuchs, N. D. Mermin and R. Schack, “An introduction to QBism with an application to the locality of quantum mechanics,”American Journal of Physics82 (2014), 749–54, arXiv: 1311.5253

work page internal anchor Pith review Pith/arXiv arXiv 2014
[12]

QBism: Quantum Theory as a Hero’s Handbook

C. A. Fuchs and B. C. Stacey, “QBism: Quantum Theory as a Hero’s Handbook.” InPro- ceedings of the International School of Physics “Enrico Fermi,” Course 197 – Foundations of Quantum Physics, edited by E. M. Rasel, W. P. Schleich and S. Wölk (Italian Physical Society, 2019).arXiv:1612.07308

work page arXiv 2019
[13]

Inadequacy of Modal Logic in Quantum Settings

N. Nurgalieva and L. del Rio, “Inadequacy of modal logic in quantum settings.” In 15th International Conference on Quantum Physics and Logic (QPL 2018),edited by G. Chiribella and P. Selinger.arXiv:1804.01106

work page internal anchor Pith review Pith/arXiv arXiv 2018
[14]

Bayesian Conditioning, the Reflection Principle, and Quantum Decoherence

C. A. Fuchs and R. Schack, “Bayesian conditioning, the reﬂection principle, and quantum decoherence,” Probability in Physics(2012), 233–47, arXiv:1103.5950

work page internal anchor Pith review Pith/arXiv arXiv 2012
[15]

B. C. Stacey,Multiscale Structure in Eco-Evolutionary Ecology.PhD thesis, Brandeis Uni- versity, 2015.arXiv:1509.02958

work page internal anchor Pith review Pith/arXiv arXiv 2015
[16]

FAQBism

J. B. DeBrota and B. C. Stacey, “FAQBism,”arXiv:1810.13401 (2018)

work page internal anchor Pith review Pith/arXiv arXiv 2018
[17]

My Struggles with the Block Universe

C. A. Fuchs,My Struggles with the Block Universe: Selected Correspondence, January 2001 – May 2011(2014), arXiv:1405.2390. See the discussions with Caves, Mermin and Schack beginning with “Bayesian Pill Taking” (p. 193)

work page internal anchor Pith review Pith/arXiv arXiv 2001

[1] [1]

The hidden assumptions of Frauchiger and Renner,

A. Sudbery, “The hidden assumptions of Frauchiger and Renner,”arXiv:1905.13248 (2019)

work page arXiv 1905

[2] [2]

Quantum theory cannot consistently describe the use of itself

D. Frauchiger and R. Renner, “Quantum theory cannot consistently describe the use of itself,” Nature Communications 9 (2018), 3711, arXiv:1604.07422

work page internal anchor Pith review Pith/arXiv arXiv 2018

[3] [3]

Remarks on the mind-body question

E. P. Wigner, “Remarks on the mind-body question.” InThe Scientist Speculates, edited by I. J. Good (Heinemann, 1961)

work page 1961

[4] [4]

Quantum probabilities as Bayesian probabilities

C. M. Caves, C. A. Fuchs and R. Schack, “Quantum probabilities as Bayesian probabilities,” Physical Review A65 (2002), 022305, arXiv:quant-ph/0106133

work page internal anchor Pith review Pith/arXiv arXiv 2002

[5] [5]

C. A. Fuchs, numerous conversations (2012–19); C. M. Caves and R. Schack, e-mail (2019)

work page 2012

[6] [6]

Subjective probability and quantum certainty

C. M. Caves, C. A. Fuchs and R. Schack, “Subjective probability and quantum certainty,” Studies in the History and Philosophy of Modern Physics 38 (2007), 255–74, arXiv: quant-ph/0608190

work page internal anchor Pith review Pith/arXiv arXiv 2007

[7] [7]

Quantum-Bayesian Coherence

C. A. Fuchs and R. Schack, “Quantum-Bayesian Coherence”arXiv:0906.2187 (2009)

work page internal anchor Pith review Pith/arXiv arXiv 2009

[8] [8]

Two dogmas about quantum mechanics

J. Bub and I. Pitowsky, “Two dogmas about quantum mechanics.” InMany Worlds?: Everett, Quantum Theory, & Reality(Oxford University Press, 2010),arXiv:0712.4258

work page internal anchor Pith review Pith/arXiv arXiv 2010

[9] [9]

Bayesian Probability Theory and Quantum Mechanics,

J. C. Baez, “Bayesian Probability Theory and Quantum Mechanics,”http://math.ucr.edu/ home/baez/bayes.html (2003)

work page 2003

[10] [10]

H. C. von Baeyer,QBism: The Future of Quantum Physics(Harvard University Press, 2016)

work page 2016

[11] [11]

An Introduction to QBism with an Application to the Locality of Quantum Mechanics

C. A. Fuchs, N. D. Mermin and R. Schack, “An introduction to QBism with an application to the locality of quantum mechanics,”American Journal of Physics82 (2014), 749–54, arXiv: 1311.5253

work page internal anchor Pith review Pith/arXiv arXiv 2014

[12] [12]

QBism: Quantum Theory as a Hero’s Handbook

C. A. Fuchs and B. C. Stacey, “QBism: Quantum Theory as a Hero’s Handbook.” InPro- ceedings of the International School of Physics “Enrico Fermi,” Course 197 – Foundations of Quantum Physics, edited by E. M. Rasel, W. P. Schleich and S. Wölk (Italian Physical Society, 2019).arXiv:1612.07308

work page arXiv 2019

[13] [13]

Inadequacy of Modal Logic in Quantum Settings

N. Nurgalieva and L. del Rio, “Inadequacy of modal logic in quantum settings.” In 15th International Conference on Quantum Physics and Logic (QPL 2018),edited by G. Chiribella and P. Selinger.arXiv:1804.01106

work page internal anchor Pith review Pith/arXiv arXiv 2018

[14] [14]

Bayesian Conditioning, the Reflection Principle, and Quantum Decoherence

C. A. Fuchs and R. Schack, “Bayesian conditioning, the reﬂection principle, and quantum decoherence,” Probability in Physics(2012), 233–47, arXiv:1103.5950

work page internal anchor Pith review Pith/arXiv arXiv 2012

[15] [15]

B. C. Stacey,Multiscale Structure in Eco-Evolutionary Ecology.PhD thesis, Brandeis Uni- versity, 2015.arXiv:1509.02958

work page internal anchor Pith review Pith/arXiv arXiv 2015

[16] [16]

FAQBism

J. B. DeBrota and B. C. Stacey, “FAQBism,”arXiv:1810.13401 (2018)

work page internal anchor Pith review Pith/arXiv arXiv 2018

[17] [17]

My Struggles with the Block Universe

C. A. Fuchs,My Struggles with the Block Universe: Selected Correspondence, January 2001 – May 2011(2014), arXiv:1405.2390. See the discussions with Caves, Mermin and Schack beginning with “Bayesian Pill Taking” (p. 193)

work page internal anchor Pith review Pith/arXiv arXiv 2001