# nLab principle of omniscience

Principles of omniscience

foundations

# Principles of omniscience

## Idea

In logic and foundations, a principle of omniscience is any principle of classical mathematics that is not valid in constructive mathematics. The idea behind the name (which is due to Bishop (1967)) is that, if we attempt to extend the computational interpretation of constructive mathematics to incorporate one of these principles, we would have to know something that we cannot compute. The main example is the law of excluded middle ($EM$); to apply $p \vee \neg{p}$ computationally, we must know which of these disjuncts hold; to apply this in all situations, we would have to know everything (hence ‘omniscience’).

Bishop's principles of omniscience (stated below) may be seen as principles that extend classical logic from predicates (where $EM$ may happen to be valid, even constructively, for certain predicates) to their quantifications over infinite domains (where $EM$ is typically not constructively valid).

## Definition

### The limited principle of omniscience

The limited principle of omniscience ($LPO$) states that the existential quantification of any decidable proposition is again decidable. That is,

$(\forall x, P(x) \vee \neg{P(x)}) \Rightarrow (\exists x, P(x)) \vee \neg(\exists x, P(x)) ,$

or equivalently

$(\forall x, P(x) \vee \neg{P(x)}) \Rightarrow (\exists x, P(x)) \vee (\forall x, \neg{P(x)}) .$

We have not stated the domain of quantification of the variable $x$. If you take it to be the subsingleton corresponding to a given truth value and apply this principle to the constantly true proposition, then $EM$ follows; conversely, $EM$ implies $LPO$ (over any domain). However, Bishop's $LPO$ takes the domain to be the set of natural numbers, giving a weaker principle than $EM$. (It appears that a realizability topos based on infinite-time Turing machines validates $LPO$ but not $EM$; see Bauer (2011).) Note that propositions of the form $\exists x, P(x)$ when $P$ is decidable are the open truth values in the Rosolini dominance.

We can also state the principle set-theoretically, with explicit reference to the domain of quantification. Given a set $A$, the limited principle of omniscience for $A$ ($LPO_A$) states that, given any function $f$ from $A$ to the boolean domain $\{0,1\}$, either $f$ is the constant map to $0$ or $1$ belongs to the image of $f$. Then Bishop's $LPO$ is $LPO_{\mathbb{N}}$, which applies to any infinite sequence of bits.

While $LPO$ for $\mathbb{N}$ is a classic example of the difference between constructive and classical mathematics, $LPO$ holds for the set $\overline{\mathbb{N}}$ of extended natural numbers; this is related to the fact that $\overline{\mathbb{N}}$ may constructively be given a compact topology. See Escardó (2011) for this and much more.

### The weak limited principle of omniscience

The weak limited principle of omniscience ($WLPO$) states that the universal quantification of any decidable proposition is again decidable. That is,

$(\forall x, P(x) \vee \neg{P(x)}) \Rightarrow (\forall x, P(x)) \vee \neg(\forall x, P(x)).$

### The lesser limited principle of omniscience

The lesser limited principle of omniscience ($LLPO$) states that, if the existential quantification of the conjunction of two decidable propositions is false, then one of their separate existential quantifications is false. That is,

$(\forall x, P(x) \vee \neg{P(x)}) \Rightarrow (\forall y, Q(y) \vee \neg{Q(y)}) \Rightarrow \neg(\exists x, P(x) \wedge \exists y, Q(y)) \Rightarrow \neg(\exists x, P(x)) \vee \neg(\exists y, Q(y)) ,$

or equivalently

$(\forall x, P(x) \vee \neg{P(x)}) \Rightarrow (\forall y, Q(y) \vee \neg{Q(y)}) \Rightarrow \neg(\exists x, P(x) \wedge \exists y, Q(y)) \Rightarrow (\forall x, \neg{P(x)}) \vee (\forall y, \neg{Q(y)}) .$

If, as before, you take the domains of quantification to be subsingletons, you get de Morgan's law $\neg(p \wedge q) \Rightarrow \neg{p} \vee \neg{q}$ ($DM$), which is weaker than $EM$; conversely, $DM$ implies $LLPO$ (over any domain). Again, Bishop's $LLPO$ takes the domain to be $\mathbb{N}$, giving a principle weaker than $DM$ (and also weaker than $LPO_{\mathbb{N}}$).

Stated set-theoretically, the lesser limited principle of omniscience for $A$ ($LLPO_A$) states that, given functions $f, g\colon A \to \{0,1\}$, if $1 \notin \im f \cap \im g$, then $1 \notin \im f$ or $1 \notin \im g$. So Bishop's $LLPO$ is $LLPO_{\mathbb{N}}$.

### Relation between the principles of omniscience

We have the following relations between the three principles of omniscience:

• $WLPO$ follows from $LPO$, but not conversely. If $P(x)$ is a decidable proposition, then so is $\neg{P(x)}$, and so $LPO$ gives

$(\exists x, \neg{P(x)}) \vee (\forall x, \neg{\neg{P(x)}}),$

which implies

$\neg(\forall x, P(x)) \vee (\forall x, P(x))$

as $P$ is decidable.

• $LLPO$ follows from $LPO$, but not conversely.

## Analytic versions

Bishop introduced the above principles of omniscience to show that certain results in pointwise analysis could not be constructive, by showing that these results implied a principle of omniscience. The principles of omniscience that appear directly in analysis are these:

• The analytic LPO states that the usual apartness relation on the set $\mathbb{R}$ of real numbers is decidable ($x \neq y$ or $x = y$), or equivalently trichotomy for the real numbers ($x \lt y$ or $x = y$ or $x \gt y$), or equivalently, that the real numbers form a discrete field.

• The analytic WLPO states that $\mathbb{R}$ has decidable equality.

• The analytic LLPO states that the usual order on $\mathbb{R}$ is a total order ($x \leq y$ or $x \geq y$), which (by analogy with trichotomy) may be called dichotomy for the real numbers.

The analytic principles of omniscience imply the corresponding ones for natural numbers; the converses hold if we assume weak countable choice (as Bishop did). In any case, if we use the modulated Cantor real numbers (sequential real numbers), then the sequential-analytic principles of omniscience are the same as those for natural numbers.

(Note that we need not accept $WCC$ to see that an analytic result implies a principle of omniscience and so cannot be constructively valid.)

## In the internal logic

In set theory, there are actually two different notions of logic: there is the external predicate logic used to define the set theory itself, and there is the internal predicate logic induced by the set operations on subsingletons and injections. In particular,

• An internal proposition is a set $P$ such that for all elements $x \in A$ and $y \in A$, $x = y$.

• The internal proposition true is a singleton $\top$.

• The internal proposition false is the empty set

• The internal conjunction of two internal propositions $P$ and $Q$ is the cartesian product $P \times Q$ of $P$ and $Q$.

• The internal disjunction of two internal propositions $P$ and $Q$ is the image of the unique function $!_{P \uplus Q}:P \uplus Q \to 1$ from the disjoint union of $P$ and $Q$ to the singleton $\top$.

$P \vee Q = \mathrm{im}(!_{P \uplus Q})$
• The internal implication of two internal propositions $P$ and $Q$ is the function set $P \to Q$ between $P$ and $Q$.

• The internal negation of an internal proposition $P$ is the function set from $P$ to the empty set

$\neg P = P \to \emptyset$
• An internal proposition $P$ is a decidable proposition if it comes with a function $\chi_P:P \to 2$ from $P$ to the boolean domain $2$.

• An internal predicate on a set $A$ is a set $P$ with injection $i:P \hookrightarrow A$, whose family of propositions indexed by $x \in A$ is represented by the preimages $i^*(x)$.

• The internal existential quantifier of an internal predicate $i:P \hookrightarrow A$ is the image of the unique function $!_P:P \to \top$ into the singleton $\top$.

$\exists_A P = \im(!_P)$
$\forall_A P = \{f \in P^A \vert \forall x \in A, f(x) \in i^*(x) \}$
• An internal predicate $i:P \hookrightarrow A$ is a decidable proposition if it comes with a function $\chi_P(x):i^*(x) \to 2$ into the boolean domain for all elements $x \in A$, or equivalently if it comes with a function $\chi_P:A \to 2$ from $A$ to the boolean domain $2$.

Then the internal LPO for a family of sets $(A_z)_{z \in I}$ is the LPO for each $A_z$ stated in the internal logic of the set theory:

• For any internal predicate $i:P \hookrightarrow A_z$, if there is a function $\chi_P:A_z \to 2$, then the internal existential quantification of $P$, $\exists_{A_z} P = \im(!_P)$ has a function $(\exists_{A_z} P) \to 2$ into the boolean domain.

or equivalently, as

• For any function $a:A_z \to 2$, the internal existential quantification of $P = \{x \in A_z \vert a(x) = 1\}$, $\exists_{A_z} P = \im(!_P)$ has a function $(\exists_{A_z} P) \to 2$ into the boolean domain.

Similarly, the internal WLPO for a family of sets $(A_z)_{z \in I}$ is the WLPO for each $A_z$ stated in the internal logic of the set theory:

• For any internal predicate $i:P \hookrightarrow A_z$, if there is a function $\chi_P:A_z \to 2$, then the internal universal quantification of $P$, $\forall_{A_z} P = \{f \in P^{A_z} \vert \forall x \in A_z, f(x) \in i^*(x) \}$ has a function $(\forall_{A_z} P) \to 2$ into the boolean domain.

or equivalently

• For any function $a:A_z \to 2$, the internal universal quantification of $P = \{x \in A_z \vert a(x) = 1\}$, $\forall_{A_z} P = \{f \in P^{A_z} \vert \forall x \in A_z, f(x) \in i^*(x) \}$ has a function $(\forall_{A_z} P) \to 2$ into the boolean domain.

And finally, the internal LLPO for a family of sets $(A_z)_{z \in I}$ is the LLPO for each $A_z$ stated in the internal logic of the set theory:

• The internal LPO for a family of sets $(A_z)_{z \in I}$ holds only for the internal predicates $i:P \hookrightarrow A_z$ which comes with an internal predicate $j:Q \hookrightarrow A_z$ such that $i^*(x) = \neg j^*(x)$ for all $x \in A_z$.

In particular, the internal LPO for the family of all subsingletons is internal excluded middle and the internal LLPO for the family of all subsingletons is internal weak excluded middle. Similarly, given a universe $U$, the internal LPO for the family of all sets in $U$ is excluded middle in $U$, and the internal LLPO for the family of all sets in $U$ is weak excluded middle in $U$.

The internal versions of the principles of omniscience, like all internal versions of axioms, are weaker than the external version of the principle of omniscience, since while bounded separation implies that one can convert any external predicate $x \in A \vdash P(x)$ on a set $A$ to an internal predicate $\{x \in A \vert P(x)\} \hookrightarrow A$, it is generally not possible to convert an internal predicate to an external predicate without a reflection rule which turns subsingletons in the set theory into propositions in the external logic.

## Truncated and untruncated versions in homotopy type theory

In the context of homotopy type theory, the various principles of omniscience can be translated in two ways, by interpreting “or” as propositionally truncated (“merely or”) or untruncated (“purely or”). The relationships between the truncated and untruncated principles of omniscience are as follows are:

## Equivalent statements

There are various other results that are equivalent to the principles of omniscience. Here are a few:

• Let $[0,1]/(0 \sim 1)$ be the quotient of the unit interval that identifies the endpoints, and let $\mathbb{R}/\mathbb{Z}$ be the quotient ring; both are classically isomorphic to the circle $\mathbb{S}^1$. (Constructively, we take $\mathbb{S}^1$ to be $\mathbb{R}/\mathbb{Z}$, although $S^1$ can also be constructed as a completion of $[0,1]/(0 \sim 1)$.) Constructively, there is an injection $[0,1]/(0 \sim 1) \hookrightarrow \mathbb{R}/\mathbb{Z}$, which is a bijection if and only if the $LLPO$ holds (for the appropriate kind of real number).

• Every semi-decidable proposition is a decidable proposition iff $\mathrm{LPO}_\mathbb{N}$ holds.

• Bolzano-Weierstrass compactness of the unit interval holds if and only if $\mathrm{LPO}_\mathbb{N}$ holds.

There are various other results that are related to the principles of omniscience. Here are a few:

• In the presence of weak countable choice, there exists a radix expansion in any base (e.g., a decimal expansion or binary expansion) for every Cauchy real number iff $\mathrm{LLPO}_\mathbb{N}$ holds; there exists a radix expansion for every Dedekind real number has iff the analytic $\mathrm{LLPO}$ holds. Without weak countable choice, Lifschitz realizability gives a model in which $\mathrm{LLPO}_\mathbb{N}$ holds but it is not true that there exists a radix expansion in any base for every Cauchy real number, which implies that there are models in which the analytic LLPO holds but it is not true that there exists a radix expansion in any base for every Dedekind real number. See Andrew Swan‘s answer to Birchfield (2024). In addition, the analytic $\mathrm{LLPO}$ holds for the Dedekind real numbers in condensed sets, but every function from the Dedekind real numbers to the boolean domain $\mathbb{2}$ is a constant function, which implies that it is not true that there exists a radix expansion in any base for every Dedekind real number.

• In the presence of countable choice, $\mathrm{LLPO}_\mathbb{N}$ is equivalent to the claim that the rings of radix expansions in any two bases are isomorphic. See Daniel Mehkeri's answer to Feldman (2010).

• That every Cauchy real number has a choice of radix expansion in any base (e.g., a decimal expansion or binary expansion) implies that the $\mathrm{WLPO}_\mathbb{N}$ holds; that every Dedekind real number has a choice of radix expansion implies that the analytic $\mathrm{WLPO}$ holds.

• Subcountability of the real numbers implies the analytic $\mathrm{WLPO}$ because natural numbers have decidable equality and injections preserve and reflect decidable equality.

• The $\mathrm{WLPO}$ implies that the real numbers are uncountably indexed.

• The analytic $\mathrm{LPO}$ implies the fundamental theorem of algebra.

## Models

• Assuming that Set is a Boolean topos, then $LPO_{\mathbb{N}}$ (the LPO for natural numbers) holds in any presheaf topos over $Set$ and indeed in any locally connected topos over $Set$, essentially since then $2^N$ is a constant object.

• The LPO for natural numbers fails in Johnstone’s topological topos, due to its internal continuity principle. Hence, the analytic LPO also fails, since the modulated Cantor reals and Dedekind reals coincide in this topos. However, the (analytic) LLPO holds, as a consequence of the preservation of finite closed unions by the inclusion of sequential spaces.

## References

The analytic WLPO and LLPO are mentioned in the following paper, although unfortunately it uses the phrase “analytic LPO” for what should really be the analytic WLPO (the reals have decidable equality):

Last revised on April 17, 2024 at 17:59:28. See the history of this page for a list of all contributions to it.