“Quantum Bayesianism (QBism) is widely regarded within the quantum-mechanics community as pseudoscientific and unscientific.”

Epistemic or QBist approaches, where the quantum state expresses an observer’s knowledge or belief, form the next most common group, followed by Everett’s many-worlds framework, in which all quantum outcomes continue to coexist without collapse (CERN Courier July/August 2025 p26).

I believe that once the notion of “agent” is precisely defined (for instance, a mentally sane Homo sapiens), QBism is a consistent and well-defined theory.

In particular, this argument was presented by T. Hänsch in his talk at the Växjö-2015 conference in which he explained why he accepts QBism as the most natural and useful interpretation of QM, especially in the light of the quantum information revolution.

In the Physics Magazine survey, Many-Worlds is the second most selected named interpretation (11%), whereas the Nature survey reported epistemic interpretations (such as Qbism) as the second most popular category (17%).

Reactions to the QBist interpretation have ranged from enthusiastic to strongly negative. … Certain authors find QBism internally self-consistent, but do not subscribe to the interpretation.

Three extra viewpoints, not in the categories provided, were identified through write-in answers: ‘QBism’, ‘Shut up and calculate’/’No interpretation needed’, and ‘None of these/unknown’.

One attempt to resolve these issues neatly is Quantum Bayesianism. Named after the 18th-century English statistician Thomas Bayes, Bayesianism is an approach to probability that’s been around theoretical physics ever since the US physicist Edwin Jaynes published a pair of papers in 1957. However, it only came to the wider attention of quantum physicists following a paper by Carlton Caves, Christopher Fuchs and Ruediger Schack in 2001 (Phys. Rev. A 65 022305). … Many physicists, though, aren’t sure about QBism. Michael Nauenberg from the University of California, Santa Cruz, for instance, has approvingly invoked Richard Feynman’s remark that, “Nature does not know what you are looking at, and she behaves the way she is going to behave whether you bother to take down the data or not”. QBists, Nauenberg charged, do not provide any experimental evidence falsifying this traditional view, nor any experiment confirming their own.

A variety of QBist and pragmatist views of quantum theory have been proposed since quantum theory assumed close to its present form. In recent years this has been an active area of research especially by philosophically aware physicists working in quantum foundations. Philosophers have tended to dismiss such approaches, objecting to their instrumentalism and/or anti-realism. But there is much to learn from responses to such objections and good philosophical reasons to take these views more seriously.

Pragmatist (Healey 2017) and subjectivist approaches—most prominently ‘QBism’ (Fuchs et al. 2014)—stick out as alternatives. These do not fall prey to the same problems, as they leave the formalism—including, and especially, the Born rule—largely intact, and refer interpretational subtleties to an informal, philosophical level.

A variety of QBist and pragmatist views of quantum theory have been proposed since quantum theory assumed close to its present form. In recent years this has been an active area of research especially by philosophically aware physicists working in quantum foundations. Philosophers have tended to dismiss such approaches, objecting to their instrumentalism and/or anti-realism. But there is much to learn from responses to such objections and good philosophical reasons to take these views more seriously.

A variety of QBist and pragmatist views of quantum theory have been proposed since quantum theory assumed close to its present form. In recent years this has been an active area of research especially by philosophically aware physicists working in quantum foundations. Philosophers have tended to dismiss such approaches, objecting to their instrumentalism and/or anti-realism. But there is much to learn from responses to such objections and good philosophical reasons to take these views more seriously.

At the V¨axj¨o-15 conference QBism was widely represented and celebrated its world-wide recognition. Nobel Prize Laure ate T. H¨ansch presented the great lecture about QBism as the only possible consistent foundational basis of quantum information theory.

The survey also revealed growing support for “epistemic” interpretations — those that treat the wavefunction not as a description of reality, but as a reflection of our knowledge or beliefs about outcomes. … One such interpretation is QBism (Quantum Bayesianism), which treats the wavefunction as personal to the observer, reflecting their subjective probabilities for measurement outcomes.

David Mermin, in his commentary in the July 2012 issue of Physics Today (page 8), put forth what he calls the QBist (quantum Bayesian) approach to quantum foundations. … Mermin has not addressed the real issue that besets quantum probabilities, he has not solved the measurement problem, and he has put the shifty split in the wrong place.

One attempt to resolve these issues neatly is Quantum Bayesianism. … Many physicists, though, aren’t sure about QBism. … Philosophy, as I’ve said many times, can often be spotted in physics wherever an issue arises that can’t be resolved by more research. QBism is one such case.

Question 12. What is your favorite interpretation of quantum mechanics? … h. Quantum Bayesianism: 6% … The Copenhagen interpretation still reigns supreme here, especially if we lump it together with intellectual offsprings such as information-based interpretations and the Quantum Bayesian interpretation.

Philosophers of physics do not agree on what is really going on here. A minority view, sometimes called “quantum Bayesianism,” says quantum theory does not tell you what is happening in nature but only what you can know about nature. This position rejects the standard philosophical position that science describes or represents the physical world.

The calculus of quantum theory is then viewed as an empirically motivated addition to Bayesian decision theory when brought to this notion of measurement. This radical approach has allowed QBism to eliminate conceptual problems that plague other interpretations of quantum mechanics.

Of course others think that we gave up on science as a discipline, because we talk about subjective degrees of belief. But we think it solves all of the foundational conundrums.

In QBism (or quantum Bayesianism) a quantum state does not represent an element of physical reality but an agentʼs personal probability assignments, reflecting his subjective degrees of belief about the future content of his experience.

The recent philosophy of Quantum Bayesianism, or QBism, rep resents an attempt to solve the traditional puzzles in the foundations of quantum theory by denying the objective reality of the quantum state. … expose what I think are weaknesses in QBism as a philosophy of science.

We are forced to conclude that the Bayesian interpretation of QM is faulted and most of its claims are not substantiated.

More and more work is done concerning quantum foundations; investigating basic properties of quantum mechanics, such as Bell’s inequality, or developing new interpretations of quantum mechanics, such as QBism. However, when one regards the results of the survey, it shows that the resurgence the topic has been undergoing in recent times still has not had an impact on the participants being familiar with foundational concepts.

A quantum foundations conference polled its 33 expert attendees about their favorite interpretation (Schlosshauer 2013). Nonrealistic interpretations, which claim that QP describes only our knowledge of the microworld rather than the microworld itself, gathered 24 votes. These sub-divided as 14 for the Copenhagen interpretation, 8 for the view that QP is about "information," and 2 for "Bayesianism" according to which quantum states represent personal degrees of belief.

The idea also forms the basis of the position (or family of positions) known as QBism (Caves et al., 2002, 2007; Fuchs and Schack, 2013; Fuchs et al., 2014; Fuchs, 2017; Fuchs and Stacey, 2019). Pro ponents of such views tend to also take quantum states to be nothing more than codifications of an agent’s beliefs.

Contemporary representatives of this tradition include those who call themselves QBists (formerly “Quantum Bayesians”) (Caves et al., 2002; Fuchs et al., 2014; Fuchs and Schack, 2015). The central tenet of QBism is that a quantum state assignment is nothing more than a way of encoding an agent’s subjective degrees of belief about that agent’s own future experiences.

QBism is one of the main candidates for an epistemic interpretation of quantum mechanics.

Quantum Bayesianism (“QBism”) has been put forward as an approach to quan tum theory that avoids foundational problems by altogether disavowing the objective existence of quantum states. … However, Fuchs’ arguments for QBism are compelling and the position sketched is nothing if not radical; it is therefore worth examining as philosophy, and searching out the physical, metaphysical and ontological assumptions underlying it.

Yet, its tenets have been questioned over the years by alternative theories or interpretations, among them Bohmian mechanics (1952), Everett’s many-worlds interpretation (1957), and various quantum-information interpretations and their offshoots, such as the ensemble interpretation, consistent histories interpretation, and quantum Bayesianism.

We give an introduction to the QBist interpretation of quantum mechanics. We note that it removes the paradoxes, conundra, and pseudo-problems that have plagued quantum foundations for the past nine decades.

Stephen Brush aptly named the issue “Wheeler’s dilemma” [36]: [H]ow can one maintain a strong version of the Copenhagen Interpretation, in which the observer is inextricably entangled with that which is observed, while at the same time denying that our consciousness affects that which we are conscious of—and thus accepting the possibility of telekinesis and other psychic effects? For Wheeler himself there is no dilemma at all; one simply has to recognize “the clear distinction between (1) the strange but well verified and repeatable features of quantum mechanics and (2) the pseudo-scientific, non-repeatable and non verified so-called extra sensory perception.” But Wheeler’s own views are likely to strike a non physicist as being just as bizarre as those of the parapsychologists he deplores. Indeed, no one has yet formulated a consistent worldview that incorporates the Copenhagen Interpretation of Quantum Mechanics while excluding what most scientists would call pseudo-sciences—astrology, parapsychology, creationism, and thousands of other cults and doctrines.

One epistemic interpretation, known as QBism (which a handful of respondents who selected ‘other’ wrote down as their preferred interpretation), takes this to the extreme, stating that observations made by a specific ‘agent’ are entirely personal and valid only for them.

QBism’s emphasis on subjective belief has led critics to dismiss it as antirealism or instrumentalism, or even, idealism or solipsism.