# Axiom:Axiom of Continuity

## Axiom Schema

This is more properly an collection of axioms rather than a single axiom.

Let $a,b,x,y,$ be points.

Let $\mathsf{B}$ be the relation of betweenness.

Let $\alpha, \beta$ be first-order formulas.

Let $\alpha$ contain no free occurrences of $a,b,y$.

Let $\beta$ contain no free occurrences of $a,b,x$.

This axiom asserts:

\(\displaystyle \) | \(\) | \(\displaystyle \exists a: \forall x,y: \left({ \alpha \land \beta \implies \mathsf{B}axy }\right)\) | |||||||||||

\(\displaystyle \) | \(\implies\) | \(\displaystyle \exists b: \forall x,y: \left({ \alpha \land \beta \implies \mathsf{B}xby }\right)\) |

### Alternative form

Let $\mathsf{B}$ be the relation of betweenness.

This axiom asserts:

- $\left({\exists a : \forall x,y : \left({x \in X \land y \in Y}\right)\implies \mathsf{B}axy}\right) \implies \left({\exists b : \forall x,y : \left({x \in X \land y \in Y}\right)\implies \mathsf{B}xby}\right)$

where $a,b,x,y$ are points and $X, Y$ are point sets.

Note that this alternate form of the **Axiom of Continuity** assumes the existence of sets.

As such, it cannot be expressed in the framework of first-order logic.

## Intuition

Consider an infinite line.

Consider it as consisting of a left side and a right side, where the transition is any place you care to choose.

No matter what place you choose, that place of transition is a point.

## Also see

## Sources

- June 1999: Alfred Tarski and Steven Givant:
*Tarski's System of Geometry*(*Bull. Symb. Log.***Vol. 5**,*no. 2*: pp. 175 – 214) : p. $185$ : Axiom $11$