Debates about "uncertainty in mathematics" and "the probabilistic turn in physics" routinely conflate three things that should be kept apart: the internal deductive relation between axioms and theorems within a formal system (which I call L-m), the approximate fit between a formal model and its target system (L-model), and the empirical or quantum probabilities of measurement outcomes (L-measure). I argue that sliding a probability or uncertainty predicate from one layer to another is a Rylean category mistake, and I supply an operational diagnostic — "probable with respect to what?" — that exposes such slides. The claim is deliberately deflationary: I do not propose a new interpretation of probability, a new measure of uncertainty, or a foundationalist defence of mathematical certainty. Rather, I offer a heuristic taxonomy whose payoff is diagnostic. I apply it to two canonical worked examples — neutrino oscillation and the double-slit experiment — and to four live objections: that the mathematics of quantum mechanics is itself probabilistic, that QBism locates probability in the agent's credence, that Gödelian incompleteness and non-standard models make mathematics itself uncertain, and that the whole distinction is merely terminological. The framework is interpretation-neutral: however one reads probability (frequentist, Bayesian, propensity, QBist), the question "probable with respect to what?" still has to be answered, and most intractable "uncertainty in mathematics" disputes dissolve once it is. The paper closes by stating the falsification conditions under which the taxonomy would lose content.
Debates about "uncertainty in mathematics" and "the probabilistic turn in physics" routinely conflate three things that should be kept apart: the internal deductive relation between axioms and theorems within a formal system (which I call L-m), the approximate fit between a formal model and its target system (L-model), and the empirical or quantum probabilities of measurement outcomes (L-measure). I argue that sliding a probability or uncertainty predicate from one layer to another is a Rylean category mistake, and I supply an operational diagnostic — "probable with respect to what?" — that exposes such slides. The claim is deliberately deflationary: I do not propose a new interpretation of probability, a new measure of uncertainty, or a foundationalist defence of mathematical certainty. Rather, I offer a heuristic taxonomy whose payoff is diagnostic. I apply it to two canonical worked examples — neutrino oscillation and the double-slit experiment — and to four live objections: that the mathematics of quantum mechanics is itself probabilistic, that QBism locates probability in the agent's credence, that Gödelian incompleteness and non-standard models make mathematics itself uncertain, and that the whole distinction is merely terminological. The framework is interpretation-neutral: however one reads probability (frequentist, Bayesian, propensity, QBist), the question "probable with respect to what?" still has to be answered, and most intractable "uncertainty in mathematics" disputes dissolve once it is. The paper closes by stating the falsification conditions under which the taxonomy would lose content.
Chapter 1: On the existence of quantum representations for two dichotomic measurements. Under which conditions do outcome probabilities of measurements possess a quantum-mechanical model? This kind of problem is solved here for the case of two dichotomic von Neumann measurements which can be applied repeatedly to a quantum system with trivial dynamics. The solution uses methods from the theory of operator algebras and the theory of moment problems. The ensuing conditions reveal surprisingly simple relations between certain quantum-mechanical probabilities. It also shown that generally, none of these relations holds in general probabilistic models. This result might facilitate further experimental discrimination between quantum mechanics and other general probabilistic theories. Chapter 2: Possibilistic Physics. I try to outline a framework for fundamental physics where the concept of probability gets replaced by the concept of possibility. Whereas a probabilistic theory assigns a state-dependent probability value to each outcome of each measurement, a possibilistic theory merely assigns one of the state-dependent labels "possible to occur" or "impossible to occur" to each outcome of each measurement. It is argued that Spekkens' combinatorial toy theory of quantum mechanics is inconsistent in a probabilistic framework, but can be regarded as possibilistic. Then, I introduce the concept of possibilistic local hidden variable models and derive a class of possibilistic Bell inequalities which are violated for the possibilistic Popescu-Rohrlich boxes. The chapter ends with a philosophical discussion on possibilistic vs. probabilistic. It can be argued that, due to better falsifiability properties, a possibilistic theory has higher predictive power than a probabilistic one. Chapter 3: The quantum region for von Neumann measurements with postselection. It is determined under which conditions a probability distribution on a finite set can occur as the outcome distribution of a quantum-mechanical von Neumann measurement with hyphenation given that the scalar product between the initial and the final state is known as well as the success probability of the postselection. An intermediate von Neumann measurement can enhance transition probabilities between states such that the error probability shrinks by a factor of up to 2. Chapter 4: A presentation of the category of stochastic matrices. This chapter gives generators and relations for the strict monoidal category of probabilistic maps on finite cardinals (i.e., stochastic matrices). Chapter 5: Convex Spaces: Definition and Examples. We try to promote convex spaces as an abstract concept of convexity which was introduced by Stone as "barycentric calculus''. A convex space is a set where one can take convex combinations in a consistent way. By identifying the corresponding Lawvere theory as the category from chapter 4 and using the results obtained there, we give a different proof of a result of Swirszcz which shows that convex spaces can be identified with algebras of a finitary version of the Giry monad. After giving an extensive list of examples of convex sets as they appear throughout mathematics and theoretical physics, we note that there also exist convex spaces that cannot be embedded into a vector space: semilattices are a class of examples of purely combinatorial type. In an information-theoretic interpretation, convex subsets of vector spaces are probabilistic, while semilattices are possibilistic. Convex spaces unify these two concepts.
Quantum mechanics is the most useful and powerful theory physicists have ever devised. Yet today, nearly 90 years after its formulation, disagreement about the meaning of the theory is stronger than ever. New interpretations appear every year. None ever disappear. Probability theory is considerably older than quantum mechanics and has also been plagued from the beginning by questions about its meaning. And quantum mechanics is inherently and famously probabilistic. For the past decade, Carl Caves, Chris Fuchs, and Rüdiger Schack have been arguing that the confusion at the foundations of quantum mechanics arises out of a confusion, prevalent among physicists, about the nature of probability [1]. They maintain that if probability is properly understood, the notorious quantum paradoxes either vanish or assume less vexing forms. Most physicists have a frequentist view of probability: Probabilities describe objective properties of ensembles of “identically prepared” systems. Caves, Fuchs, and Schack take a personalist Bayesian view: An agent assigns a probability p to a single event as a measure of her belief that the event will take place [2]. Such an agent is willing to pay less than $ p for a coupon that will pay her $1 if the event happens, and she is willing to underwrite and sell such a coupon for more than $ p . Surprisingly, the standard rules for probability follow from the requirement that an agent should never face certain loss in a single event. (For example, if p exceeded 1, she would pay more than $1 for a coupon that returned at most $1; if p were negative, she would pay somebody to take a coupon from her that might cost her another $1.) Avoiding certain loss is the only constraint on an agent's probability assignments. The probability of an event is not inherent in that event. Different agents, with different beliefs, will in general assign different probabilities to the same event. The personalist Bayesian view of probability is widely held [3], though not by many physicists. It has profound implications for the meaning of quantum mechanics, which Fuchs and Schack call quantum Bayesianism—QBism for short. Since quantum states determine probabilities, if probabilities are indeed assigned by an agent to express her degree of belief, then the quantum state of a physical system is not inherent in that system but assigned by an agent to encapsulate her beliefs about it. State assignments, like probabilities, are relative to an agent.
In the QBist approach to quantum mechanics, a measurement is an action an agent takes on the world external to herself. A measurement device is an extension of the agent and both measurement outcomes and their probabilities are personal to the agent. According to QBism, nothing in the quantum formalism implies that the quantum state assignments of two agents or their respective measurement outcomes need to be mutually consistent. Recently, Khrennikov has claimed that QBism's personalist theory of quantum measurement is invalidated by Ozawa's so-called intersubjectivity theorem. Here, following Stacey, we refute Khrennikov's claim by showing that it is not Ozawa's mathematical theorem but an additional assumption made by Khrennikov that QBism is incompatible with. We then address the question of intersubjective agreement in QBism more generally. Even though there is never a necessity for two agents to agree on their respective measurement outcomes, a QBist agent can strive to create conditions under which she would expect another agent's reported measurement outcome to agree with hers. It turns out that the assumptions of Ozawa's theorem provide an example for just such a condition.
QBists insist that theirs is a realist position while denying that the quantum formalism represents a mind-independent world. The insistence is hard to place. The realism debate is throughout a dispute about represented content, so a theory that denies standing in the representation relation appears not to take up a position within that debate at all. This paper examines the most developed attempt to make good on the QBist claim--David Glick's perspectival normative realism--and argues that it fails instructively. Glick seeks the ground of the Born rule's objectivity among states of affairs and construes an agent's perspective as a location within an already-constituted world; finding no state of affairs fit to bind every agent at once, he must book the ground as brute. Phenomenology supplies the third thing his picture lacks. A horizon is not a position within the world but the condition under which a world is given. Read as an agent's weighting--her curvature--of such a horizon, the quantum state is neither a defective description nor a brute posit, and the Born rule articulates the invariant lawfulness by which any sense confers a curvature at all.
This is the eleventh paper of a series that redescribes physical reality as a causally consistent history of information updates, and the second panel of its second triptych — observer, then probability and typicality, then epistemic trust — ordered by dependence. The tenth paper defined a record-bearing observer as an operationally individuated physical subsystem that acquires and integrates records, decodes them through an implemented internal channel, and lets the decoded content shape later processing. It defined that subject but did not say how to count or weigh it, and it handed forward five unresolved demands: plural individuation, the observer-grain problem, temporal segmentation, one-to-many lineage continuation, and the fact that a Boltzmann brain is not excluded from observerhood. This paper takes up the weighing. It does not propose the correct cosmological measure; it asks what must be specified before a sentence about typical observers or self-locating credence is even well posed. The contribution is a measure-specification problem, not a measure. Two theses organize the answer. First, the several things called “probability” — dynamical transition probability, the Born weight, empirical frequency, an observer selection measure, self-locating credence, and normative trust — are conceptually and type-theoretically distinct, and identifying any one numerical assignment with another requires an explicit theoretical or interpretive bridge rather than following automatically. Second, a third-personal typicality claim and a first-personal self-locating credence claim are specified separately, the former needing no self-location bridge. Formally, history is the base measure layer, weighted within a fixed decoherent-history framework; observer architectures, observation episodes, observer-moments, record-derived states, and lineages are within-history fiber types, weighted by a non-negative measure kernel concentrated on a type-specific eligible set. The base measure and the kernel are separately specified and independently motivated inputs whose factorization cannot be recovered from the final observer measure alone. Typicality is a reference-class conditional prevalence; self-locating credence requires a further bridge that reads a physical or selectional measure as first-person credence; a conditioning rule for a measure-zero conditioning event is distinguished from, and ordered before, the regularization prescription for an infinite measure; and causal provenance, veridicality, and channel reliability are kept apart, with provenance prevalence treated as a diagnostic, not a verdict. The account is positioned explicitly relative to the xerographic-distribution program of Hartle and Srednicki. The paper makes no independent empirical prediction; it derives neither quantum theory, the Born rule, nor a unique cosmological measure, and it offers no normative theory of trust. It supplies a typed specification, the minimal conditions under which a typicality statement carries meaning, and a hierarchical specification family over which the twelfth paper’s self-undermining-trust criterion can range.
There seems to be nothing short of a {\it double whammy} hitting the users of probability, and among them physicists, especially those involved in the foundations of quanta. First is the instant instinctual reaction that phenomena which interest one do sharply and clearly divide into the {\it dichotomy} of {\it two and only two} alternatives of being {\it either} ``deterministic", {\it or} on the contrary, being ``probabilistic". However, there is also a second, prior and yet deeper trouble, namely, the ``probabilistic" case is strongly believed to be equally clear and well-founded as is the ``deterministic" one. And the only difference seen between the two is that the latter can talk also about ``individual" phenomena, while the former can only do so about large enough ``ensembles" for which, however, it is believed to be equally clear, precise and rigorous with the ``deterministic" approach. Or briefly, ``probabilistic'' is seen as nothing else but the ``deterministic'' on the level of ``ensembles" ... \\The fact, however, is that there is a {\it deep gap} between the empirical world of ``random" phenomena, and on the other hand, theories of ``probability". Furthermore, any attempt to bridge that gap does inevitably involve {\it infinity}, thus aggravating the situation to the extent that even today, and even if not quite realized by many, theories of ``probabilities" have a {\it shaky} foundation. \\This paper tries to bring to the awareness of various users of ``probabilities", and among them, to physicists involved in quanta, the fact that - seemingly unknown to them - they are self-inflicted victims of the mentioned double whammy. \\ \\