Russell’s paradox

Content created by Fredrik Bakke.

Created on 2024-10-09.
Last modified on 2024-10-09.

{-# OPTIONS --lossy-unification #-}

module set-theory.russells-paradox where
Imports 
open import foundation.dependent-pair-types
open import foundation.functoriality-cartesian-product-types
open import foundation.identity-types
open import foundation.locally-small-types
open import foundation.negation
open import foundation.small-types
open import foundation.small-universes
open import foundation.surjective-maps
open import foundation.torsorial-type-families
open import foundation.type-arithmetic-cartesian-product-types
open import foundation.type-arithmetic-dependent-pair-types
open import foundation.universal-property-equivalences
open import foundation.universe-levels

open import foundation-core.contractible-types
open import foundation-core.empty-types
open import foundation-core.equivalences
open import foundation-core.functoriality-dependent-pair-types

open import trees.multisets
open import trees.small-multisets
open import trees.universal-multiset

Idea

Russell’s paradox arises when a set of all sets is assumed to exist. In Russell’s paradox it is of no importance that the elementhood relation takes values in propositions. In other words, Russell’s paradox arises similarly if there is a multiset of all multisets. We will construct Russell’s paradox from the assumption that a universe 𝒰 is equivalent to a type A : 𝒰. We conclude that there can be no universe that is contained in itself. Furthermore, using replacement we show that for any type A : 𝒰, there is no surjective map A → 𝒰.

Definition

Russell’s multiset

Russell : (l : Level) → 𝕍 (lsuc l)
Russell l =
  comprehension-𝕍
    (universal-multiset-𝕍 l)
    (λ X → X ∉-𝕍 X)

Properties

If a universe is small with respect to another universe, then Russells multiset is also small

module _
  {l1 l2 : Level} (H : is-small-universe l2 l1)
  where

  is-small-Russell : is-small-𝕍 l2 (Russell l1)
  is-small-Russell =
    is-small-comprehension-𝕍 l2
      { lsuc l1}
      { universal-multiset-𝕍 l1}
      { λ z → z ∉-𝕍 z}
      ( is-small-universal-multiset-𝕍 l2 H)
      ( λ X → is-small-∉-𝕍 l2 (K X) (K X))
    where
    K = is-small-multiset-𝕍 (pr2 H)

  resize-Russell : 𝕍 l2
  resize-Russell = resize-𝕍 (Russell l1) (is-small-Russell)

  is-small-resize-Russell :
    is-small-𝕍 (lsuc l1) (resize-Russell)
  is-small-resize-Russell =
    is-small-resize-𝕍 (Russell l1) (is-small-Russell)

  equiv-Russell-in-Russell :
    (Russell l1 ∈-𝕍 Russell l1) ≃ (resize-Russell ∈-𝕍 resize-Russell)
  equiv-Russell-in-Russell =
    equiv-elementhood-resize-𝕍 (is-small-Russell) (is-small-Russell)

Russell’s paradox obtained from the assumption that 𝒰 is 𝒰-small

paradox-Russell : {l : Level} → ¬ (is-small l (UU l))
paradox-Russell {l} H =
  no-fixed-points-neg
    ( R ∈-𝕍 R)
    ( pair (map-equiv β) (map-inv-equiv β))
  where

  K : is-small-universe l l
  K = pair H (λ X → pair X id-equiv)

  R : 𝕍 (lsuc l)
  R = Russell l

  is-small-R : is-small-𝕍 l R
  is-small-R = is-small-Russell K

  R' : 𝕍 l
  R' = resize-Russell K

  is-small-R' : is-small-𝕍 (lsuc l) R'
  is-small-R' = is-small-resize-Russell K

  abstract
    p : resize-𝕍 R' is-small-R' ＝ R
    p = resize-resize-𝕍 is-small-R

  α : (R ∈-𝕍 R) ≃ (R' ∈-𝕍 R')
  α = equiv-Russell-in-Russell K

  abstract
    β : (R ∈-𝕍 R) ≃ (R ∉-𝕍 R)
    β = ( equiv-precomp α empty) ∘e
        ( ( left-unit-law-Σ-is-contr
            { B = λ t → (pr1 t) ∉-𝕍 (pr1 t)}
            ( is-torsorial-Id' R')
            ( pair R' refl)) ∘e
          ( ( inv-associative-Σ (𝕍 l) (_＝ R') (λ t → (pr1 t) ∉-𝕍 (pr1 t))) ∘e
            ( ( equiv-tot
                ( λ t →
                  ( commutative-product) ∘e
                  ( equiv-product
                    ( id-equiv)
                    ( inv-equiv
                      ( ( equiv-concat'
                          _ ( p)) ∘e
                        ( eq-resize-𝕍
                          ( is-small-multiset-𝕍 is-small-lsuc t)
                          ( is-small-R'))))))) ∘e
              ( associative-Σ
                ( 𝕍 l)
                ( λ t → t ∉-𝕍 t)
                ( λ t →
                  ( resize-𝕍
                    ( pr1 t)
                    ( is-small-multiset-𝕍 is-small-lsuc (pr1 t))) ＝
                  ( R))))))

There can be no surjective map f : A → 𝒰 for any A : 𝒰

no-surjection-onto-universe :
  {l : Level} {A : UU l} (f : A → UU l) → ¬ (is-surjective f)
no-surjection-onto-universe f H =
  paradox-Russell
    ( is-small-is-surjective H
      ( is-small')
      ( is-locally-small-UU))

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