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A characterisation of ordered abstract probabilities

by   Abraham Westerbaan, et al.

In computer science, especially when dealing with quantum computing or other non-standard models of computation, basic notions in probability theory like "a predicate" vary wildly. There seems to be one constant: the only useful example of an algebra of probabilities is the real unit interval. In this paper we try to explain this phenomenon. We will show that the structure of the real unit interval naturally arises from a few reasonable assumptions. We do this by studying effect monoids, an abstraction of the algebraic structure of the real unit interval: it has an addition x+y which is only defined when x+y≤ 1 and an involution x 1-x which make it an effect algebra, in combination with an associative (possibly non-commutative) multiplication. Examples include the unit intervals of ordered rings and Boolean algebras. We present a structure theory for effect monoids that are ω-complete, i.e. where every increasing sequence has a supremum. We show that any ω-complete effect monoid embeds into the direct sum of a Boolean algebra and the unit interval of a commutative unital C^*-algebra. This gives us from first principles a dichotomy between sharp logic, represented by the Boolean algebra part of the effect monoid, and probabilistic logic, represented by the commutative C^*-algebra. Some consequences of this characterisation are that the multiplication must always be commutative, and that the unique ω-complete effect monoid without zero divisors and more than 2 elements must be the real unit interval. Our results give an algebraic characterisation and motivation for why any physical or logical theory would represent probabilities by real numbers.


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I Introduction

Probability theory in the quantum realm is different in important ways from that of the classical world. Nevertheless, they both crucially rely on real numbers to represent probabilities of events. This makes sense as observations of quantum systems must still be interpreted trough classical means. However, in principle one can imagine a world governed by different physical laws where even the standard notion of a probability is different, or wish to study probabilistic models where one does not care about the specifics of their probabilities; such an approach can for instance be found in categorical quantum mechanics [abramsky2004categorical, gogioso2017fantastic, tull2016reconstruction]. In this paper we study a reasonable class of alternatives to the real unit interval as the set of allowed probabilities. We will establish that this quite general seeming class actually only contains (continuous products of) the real unit interval. This shows that any ‘reasonable’ enough physical theory must necessarily be based on probabilities represented by real numbers.

In order to determine the right set of alternatives to the real unit interval we must first find out what structure is crucial for abstract probabilities. There are a variety of operations on the real unit interval that are used in their interpretation as probabilities. First of all, to be able to talk about coarse-graining the probabilities of mutually exclusive events, we must be able to take the sum of two probabilities as long as . Second, in order to represent the complement of an event we require the involution given by . The probability is the unique number such that . Axiomatising this structure of a partially defined addition combined with an involution defines an effect algebra [foulis1994effect]. The unit interval of course also has a multiplication

. This operation is needed in order to talk about, for instance, joint distributions. An

effect monoid is an effect algebra with an associative distributive (possibly non-commutative) multiplication, and hence axiomatises these three interacting algebraic structures (addition, involution and multiplication) present in the unit interval.

In order to define an analogue to Bayes’ theorem we would also need the division operation that is available in the unit interval: when

, then there is a probability such that (namely ). We actually will not require the existence of such a division operation, as it turns out to follow (non-trivially) from our final requirement:

A property that sets the unit interval apart from, for instance, the rational numbers between and , is that is closed under taking limits. In particular, each ascending chain of probabilities has a supremum. In other words: the unit interval is -complete. We have then arrived at our candidate for an abstract notion of the set of probabilities: an -complete effect monoid.

Further motivation for the use of this structure as a natural candidate for the set of probabilities is its prevalence in effectus theory. This is a recent approach to categorical logic [cho2015introduction] and a general framework to deal with notions such as states, predicates, measurement and probability in deterministic, (classical) probabilistic and quantum settings [kentathesis, basthesis]. The set of probabilities in an effectus have the structure of an effect monoid. Examples of effectuses include any generalised probabilistic theory [barrett2007information], where the probabilities are the unit interval, but also any topos (and in fact any extensive category with final object), where the probabilities are the Boolean values  [kentathesis].

An effectus defines the sum of some morphisms. In a -effectus, this is strengthened to the existence of some countable sums (making it a partially additive category [arbib1980partially]). In such an effectus the probabilities form an -complete effect monoid [kentathesis].

Effect monoids are of broader interest than only to study effectuses: examples of effect monoids include all Boolean algebras and unit intervals of partially ordered rings. Furthermore any effect monoid can be used to define a generalised notion of convex set and convex effect algebra (by replacing the usual unit interval by elements of the effect monoid, see [basthesis, 179 & 192] or [jacobs2011probabilities]).

This now raises the question of how close a probability theory based on an -complete effect monoid is to regular probability theory.

Our main result is that -complete effect monoids can always be embedded into a direct sum of an -complete Boolean algebra and the unit interval of a bounded--complete commutative unital C-algebra. The latter is isomorphic to  for some basically disconnected compact Hausdorff space . If the effect monoid is directed complete, such thay any directed set has a supremum, then it is even isomorphic to the direct sum of a complete Boolean algebra and the unit interval of a monotone complete commutative C-algebra.

This result basically states that any -complete effect monoid can be split up into a sharp part (the Boolean algebra), and a convex probabilistic part (the commutative C*-algebra). This then gives us from basic algebraic and order-theoretic considerations a dichotomy between sharp and fuzzy logic.

As part of the proof of this embedding theorem we find an assortment of additional structure present in -complete effect monoids: it has a partially defined division operation, it is a lattice, and multiplication must necessarily be normal (i.e. preserve suprema).

The classification also has some further non-trivial consequences. In particular, it shows that any -complete effect monoid must necessarily be commutative.

Finally, we use the classification to show that an -complete effect monoid without zero divisors must either be trivial, , the two-element Boolean algebra, , or the unit interval, . This gives a new characterisation of the real unit interval as the unique -complete effect monoid without zero divisors and more than two elements, and could be seen as a generalisation of the well-known result that the set of real numbers is the unique Dedekind-complete Archimedian ordered field.

In so far as the structure of an -complete effect monoid is required for common actions involving probabilities, (coarse-graining, negations, joint distributions, limits) our results motivate the usage of real numbers in any hypothetical alternative physical theory.

Ii Preliminaries

Before we state the main results of this paper technically, we recall the definitions of the structures involved.

Definition 1.

An effect algebra (EA) is a set  with distinguished element , partial binary operation  (called sum) and (total) unary operation  (called complement), satisfying the following axioms, writing  whenever  is defined and .

  • Commutativity: if , then and .

  • Zero: and .

  • Associativity: if and , then , , and .

  • For any , the complement is the unique element with .

  • If for some , then .

For  we write  whenever there is a  with . This turns  into a poset with minimum  and maximum . The map  is an order anti-isomorphism. Furthermore  if and only if . If , then the element  with  is unique and is denoted by  [foulis1994effect].

A morphism  between effect algebras is a map such that  and  whenever , and then . A morphism necessarily preserves the complement, , and the order: . A morphism is an embedding when it is also order reflecting: if then . Observe that an embedding is automatically injective. We say and are isomorphic and write when there exists an isomorphism (i.e. a bijective morphism whose inverse is a morphism too) from to . Note that an isomorphism is the same as a surjective embedding.

Example 2.

Let be an orthomodular lattice. Then is an effect algebra with the partial addition defined by and in that case . The complement, , is given by the orthocomplement, . The lattice order coincides with the effect algebra order (defined above). See e.g. [basmaster, Prop. 27].

Example 3.

Let  be an ordered abelian group (such as the self-adjoint part of a C-algebra). Then any interval where  is a positive element of  forms an effect algebra, with addition given by and in that case . The complement is defined by . The effect algebra order on  coincides with the regular order on .

In particular, the set of effects of a unital C-algebra  forms an effect algebra with , and .

Effect algebras have been studied extensively (to name a few: [chajda2009every, ravindran1997structure, gudder1996examples, gudder1996effect, jenvca2001blocks, dvurevcenskij2010every, foulis2010type]) and even found surprising applications in quantum contextuality  [staton2018effect, roumen2016cohomology] and the study of Lebesque integration [jacobs2015effect]. The following remark gives some categorical motivation to the definition of effect algebras.

Remark 4.

An effect algebra is a bounded poset: a partially ordered set with a minimal and maximal element. In [mayet1995classes] it is shown that any bounded poset can be embedded into an orthomodular poset . This is known as the Kalmbach extension [kalmbach1977orthomodular]. This extends to a functor from the category of bounded posets to the category of orthomodular posets, and this functor is in fact left adjoint to the forgetful functor going in the opposite direction [harding2004remarks]. This adjunction gives rise to the Kalmbach monad on the category of bounded posets. The Eilenberg–Moore category for the Kalmbach monad is isomorphic to the category of effect algebras, and hence effect algebras are in fact algebras over bounded posets [jenvca2015effect].

The category of effect algebras is both complete and cocomplete. There is also an algebraic tensor product of effect algebras that makes the category of effect algebras symmetric monoidal 

[jacobs2012coreflections]. The monoids in the category of effect algebras resulting from this tensor product are called effect monoids, and they can be explicitly defined as follows:

Definition 5.

An effect monoid (EM) is an effect algebra with an additional (total) binary operation , such that the following conditions hold for all .

  • Unit: .

  • Distributivity: if , then , ,

    Or, in other words, the operation  is bi-additive.

  • Associativity: .

We call an effect monoid  commutative if for all ; an element of  idempotent whenever ; elements , of  orthogonal when ; and we denote the set of idempotents of by .

Example 6.

Any Boolean algebra , being an orthomodular lattice, is an effect algebra by Example 2, and, moreover, a commutative effect monoid with multiplication defined by . Conversely, any orthomodular lattice for which  distributes over  (and thus ) is a Boolean algebra.

Example 7.

The unit interval of any (partially) ordered unital ring  (in which the sum and product of positive elements  and  are again positive) is an effect monoid.

Let, for example, be a compact Hausdorff space. We denote its space of continuous functions into the complex numbers by . This is a commutative unital C-algebra (and conversely by the Gel’fand theorem, any commutative C-algebra with unit is of this form) and hence its unit interval is a commutative effect monoid.

In [kentathesis, Ex. 4.3.9] and [basmaster, Cor. 51] two different non-commutative effect monoids are constructed.

Definition 8.

Let and be effect monoids. A morphism from to is a morphism of effect algebras with the added condition that  for all . Similar to the case of effect algebras, an embedding  is a morphism that is order reflecting. Also here an isomorphism of effect monoids is the same thing as a surjective embedding of effect monoids.

Example 9.

It is well-known that any Boolean algebra is isomorphic to the set of clopens of its Stone space . This yields an effect monoid embedding from into .

Remark 10.

A physical or logical theory which has probabilities of the form can be seen as a theory with a natural notion of space, where probabilities are allowed to vary continuously over the space . Such a spatial theory is considered in for instance Ref. [moliner2017space].

Example 11.

Given two effect algebras/monoids  and we define their direct sum as the Cartesian product with pointwise operations. This is again an effect algebra/monoid.

Example 12.

Let  be an effect monoid and let  be some idempotent. The subset  is called the left corner by  and is an effect monoid with  and all other operations inherited from . Later we will see that  is an isomorphism . Analogous facts hold for the right corner .

Definition 13.

Let be an effect algebra. A directed set is a non-empty set such that for all there exists a such that . is directed complete when for any directed set there is a supremum . It is -complete if directed suprema of countable sets exist, or equivalently if any increasing sequence in has a supremum.

Remark 14.

A directed complete partially ordered set is often referred to by the shorthand dcpo. These structures lie at the basis of domain theory and are often encountered when studying denotational semantics of programming languages as they allow for a natural way to talk about fix points of recursion. Note that being -complete is strictly weaker. For effect algebras we could have equivalently defined directed completeness with respect to downwards directed sets, as the complement is an order anti-isomorphism.

Example 15.

Let  be a -complete Boolean algebra. Then  is a -complete effect monoid. If  is complete as a Boolean algebra, then  is directed-complete as effect monoid.

Example 16.

Let be an extremally disconnected compact Hausdorff space, i.e. where the closure of every open set is open. Then is a directed-complete effect monoid. If  is a basically disconnected [gillman2013rings, 1H] compact Hausdorff space, i.e. where every cozero set has open closure, then  is an -complete effect monoid [gillman2013rings, 3N.5].

Iii Overview

The main results of the paper are the following theorems:


Let be an -complete effect monoid. Then embeds into , where is an -complete Boolean algebra, and , where is a basically disconnected compact Hausdorff space (see Theorem 68).


Let be a directed-complete effect monoid. Then where is a complete Boolean algebra and for some extremally-disconnected compact Hausdorff space  (see Theorem 69).

By Example 9, the Boolean algebra also embeds into a , and hence we could ‘coarse-grain’ the direct sums above and say that any -complete effect monoid embeds into the unit interval of , where is the disjoint union of the topological spaces. This observation suggests a Stone-type duality that we discuss in more detail in the conclusion.

Other results for an -complete effect monoid that either follow directly from the above theorems, or are proven along the way are the following:

  • is a lattice.

  • is an effect divisoid [basthesis].

  • The multiplication in is normal: .

  • If  is convex (as an effect algebra), then scalar multiplication is homogeneous: for any  and .

  • is commutative.

  • If has no non-trivial zero-divisors (i.e. , implies or ), then is isomorphic to , or .

It should be noted that the scalars in a -effectus satisfying normalisation have no non-trivial zero-divisors [kentathesis] and hence using the last point above, we have completely characterised the scalars in such -effectuses, splitting them up into trivial, Boolean and convex effectuses.

The paper is structured as follows. In Section IV we recover and prove some basic results regarding effect algebras/monoids. Then in Section V we will show that in any -complete effect monoid , we can define a kind of partial division operation which turns it into a effect divisoid. Using this division we show that the multiplication must be normal. Then in Section VI we study idempotents that are either Boolean, meaning that all elements below must also be idempotents, or halvable, meaning that there is an such that . We establish that an -complete effect monoid where is Boolean must be a Boolean algebra, while if is halvable then it must be convex. In Section VII we show that a maximal collection of orthogonal idempotents of  can be found that consists of a mix of halvable and Boolean idempotents. The corner associated to such an idempotent will either be convex (if  is halvable) or Boolean (if  is Boolean). Using normality of multiplication we show that embeds into the direct sum of the corners associated to these idempotents. Letting  be the direct sum of the Boolean corners, and  be the direct sum of the convex corners, we see that  embeds into , where  is Boolean and  is convex. In Section VIII, we recall some results regarding order unit spaces and use Yosida’s representation theorem to show that a convex -complete effect monoid must be isomorphic to the unit interval of a . Then in Section IX we collect all the results and prove our main theorems. Finally in Section X we conclude and discuss some future work and open questions.

Iv Basic results

We do not assume any commutativity of the product in an effect monoid. Nevertheless, some commutativity comes for free.

Lemma 17.

For any  in an effect monoid , we have .


. Cancelling on both sides gives the desired equality. ∎

Lemma 18.

An element is an idempotent if and only if .


. Hence if and only if . ∎

Lemma 19.

For  with , we have


Assume . Then , so that . Similarly . Hence . Similarly .

Now assume . Then immediately . The final implication (that ) is proven similarly. ∎

Lemma 20.

Let be an effect monoid with idempotent . Then  for any .


Clearly  and so by Lemma 19 . Similarly  and so . Thus , as desired. ∎

Corollary 21.

Let  be an effect monoid with idempotent . The map  is an isomorphism .

The following two lemmas are simple observations that will be used several times.

Lemma 22.

Let  be elements of an effect algebra . If  for some  from , then  (and ).


Since  and , we have , and so . Then , yielding and , so  and . ∎

Lemma 23.

Let  be an idempotent from an effect monoid , and let  be elements below . If  exists, then .


Since , we have , and similarly, . But then , and hence . By Lemma 19 we then have . ∎

We defined directed set to mean upwards directed. Using the fact that is an order anti-isomorphism, a directed-complete effect algebra also has all infima of downwards directed (or ‘filtered’) sets (and similarly for countable infima in a -complete effect algebra).

Recall that given an element  of an ordered group  a subset  of  has a supremum in  if and only if exists, which follows immediately from the observation that  is an order isomorphism. For effect algebras the situation is a bit more complicated, and we only have the implications mentioned in the lemma below. We will see in Corollary 39 that the situation improves somewhat for -complete effect monoids.

Lemma 24.

Let  be an element and a non-empty subset of an effect algebra . If , then

Here “” means also that the sums, suprema and infima on either side exist. Similarly, if , then
Moreover, if , then

Note that gives an order isomorphism  with inverse . Whence  preserves and reflects all infima and suprema restricted to  and . Surely, given elements from , and a subset  of the interval , it is clear that any supremum (infimum) of  in  will be the supremum (infimum) of  in  too (using here that  is non-empty). The converse does not always hold, but when has a supremum in , then this is the supremum in  too (and when  has an infimum in , then this is the infimum in  too). These considerations yield the first four equations. For the latter two we just add the observation that gives an order reversing isomorphism . ∎

We can now prove a few basic yet useful facts of -complete effect monoids. These lemmas deal with elements that are summable with themselves: elements such that which means that is defined. For we will use the notation for the -fold sum of with itself (when it is defined). We study these self-summable elements to be able to define a “” in some effect monoids later on.

Lemma 25.

For any  in some effect monoid , the element  is summable with itself.


Since , and by Lemma 17, we see that  indeed exists. ∎

Lemma 26.

Let be an element of an -complete effect monoid .

  1. If exists for all then .

  2. If then .

  3. If then .


For point 1, we have , and so .

For point 2, since we have , and hence (because of Lemma 25) is summable with itself. But furthermore , and so . Continuing in this fashion, we see that  exists for every  and . Hence, for any  the sum  exists so that by the previous point .

For point 3, write . As and we see that is defined. But this is true for all , and so again by the point 1, . ∎

V Floors, ceilings and division

In this section we will see that any -complete effect monoid has floors and ceilings. These are respectively the largest idempotent below an element and the smallest idempotent above an element. We will also construct a “division”: for we will find an element such that .

Then using ceilings and this division we will show that multiplication in a -complete effect monoid is always normal, i.e. that for non-empty  for which  exists. This technical result will be frequently used in the remaining sections.

Definition 27.

Let  be a (potentially infinite) family of elements from an effect algebra . We say that the sum  exists if for every finite subset  the sum  exists and the supremum  exists as well. In that case we write .

Lemma 28.

Given  for an effect monoid , we have

for every natural number .


From the computation

the result follows immediately. ∎

Corollary 29.

The sum  exists for any element  from an -complete effect monoid .

Definition 30.

Given an element  of an -complete effect monoid 

are called the ceiling of a and the floor of , respectively.

We list some basic properties of  and  in Proposition 35, after we have made the observations necessary to establish them.

Lemma 31.

Given an element  of an -complete effect monoid , we have .


Write . Since  and  commute by Lemma 17, we compute

and in particular see that the sum exists. Because , the -fold sum  exists too and hence  by Lemma 26. ∎

Lemma 32.

We have  for any element  of an -complete effect monoid .


Using Lemmas 17 and 31 we compute , and so . The other identity has a similar proof. ∎

Lemma 33.

Given elements of a -complete effect monoid  such that  exists, and  for all , we have .


Writing , we have  and  for all . Since  for all , we have

which implies that , and thus . ∎

Proposition 34.

Given elements and  of an -complete effect monoid ,


If , then also for all . Hence by Lemma 33 . ∎

Proposition 35.

Let  be an element of an -complete effect monoid .

  1. The floor  of  is an idempotent with . In fact, is the greatest idempotent below .

  2. The ceiling  of  is the least idempotent above .

  3. We have and .


Point 3 follows from Lemma 28. Concerning point 1: Since (by Lemma 32) we have by Proposition 34, and so because by point 3. Hence is an idempotent. Also, since , we clearly have . Now, if  is an idempotent in  with , then , and so . Whence  is the greatest idempotent below . Point 2 now follows easily from 1, since  is the dual of  under the order anti-isomorphism . ∎

Lemma 36.

for all summable elements and  of an -complete effect monoid  (that is, is the supremum of  and ).


Since , we have , and similarly, . Let  be an upper bound of  and ; we claim that . Since  and , we have and , and so  by Lemma 23. Whence . ∎

Any -complete effect monoid is a lattice effect algebra [rievcanova2000generalization]:

Theorem 37.

Any pair of elements and from an -complete effect monoid  has an infimum, , given by

Consequently, any pair also has a supremum given by .


First order of business is showing that the sum  exists for every . In fact, we’ll show that for all , by induction. Indeed, for , we have , and if for some , then . In particular, exists, and, moreover,

By a similar reasoning, we get

Already writing , we know at this point that and . It remains to be shown that  defined above is the greatest lower bound of  and . So let  with and  be given; we must show that .

As an intermezzo, we observe that . Indeed, we have , and , because  exists (see Lemma 26). By Proposition 34 it follows that . Whence writing , we have using Lemma 20. Observing that and using Lemma 19 we also have . We then calculate and similarly .

Returning to the problem of whether , we have

Whence  is the infimum of  and . ∎

The presence of finite infima and suprema in -complete effect monoids prevents certain subtleties around the existence of arbitrary suprema and infima.

Corollary 38.

Let  be elements of an -complete effect monoid , and let be a non-empty subset of .

Then  has a supremum (infimum) in  if and only if  has a supremum (infimum) in , and these suprema (infima) coincide.


It is clear that if  has a supremum in , then this is also the supremum in . For the converse, suppose that  has a supremum  in , and let  be an upper bound for  in ; in order to show that  is the supremum of  in  too, we must prove that . Note that  is an upper bound for . Indeed, given  we have , and , so . Moreover, one easily sees that using the fact that  is non-empty. Whence  is an upper bound for  in , and so , making  the supremum of  in . Similar reasoning applies to infima of . ∎

Corollary 39.

Given an element  and a non-empty subset  of an -complete effect monoid  such that exists for all ,

  1. the supremum  exists iff exists, and in that case ;

  2. the infimum  exists iff exists, and in that case .


The map , being an order isomorphism, preserves and reflects suprema and infima. Now apply Corollary 38. ∎

Now that we know more about the existence of suprema and infima, we set our sights on proving that multiplication interacts with suprema and infima as desired, namely that it preserves them. To do this we introduce a partial division operation.

Definition 40.

Given elements  of an -complete effect monoid, set

Note that the sum exists, because for all .

Lemma 41.

Let be an element of an -complete effect monoid .

  1. .

  2. for all summable  with .

  3. for all .

  4. for all  with .

  5. .

  6. The maps are order isomorphisms.


Points 1 and 2 are easy, and left to the reader. Concerning 3, first note that

Thus . Since similarly , we get, using 2,

forcing  (see Lemma 22). For point 4, note that given  with  we have (by Lemma 19, since ,) and so

by point 3