Axiom:Euclid's Axiom
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Axiom
First Form
Let $\mathsf{B}$ be the relation of betweenness.
Let $=$ be the relation of equality.
This axiom asserts that:
- $\forall a, b, c, d, t: \exists x, y:$
- $\left({\mathsf{B}adt \land \mathsf{B}bdc \land \neg (a = d)}\right) \implies \left({\mathsf{B}abx \land \mathsf{B}acy \land \mathsf{B}xty}\right)$
where $a, b, c, d, t, x, y$ are points.
Intuition
Draw an angle $bac$ such that $0^{\circ} \le \angle{bac} \le 180^{\circ}$
Let $t$ be some point in the interior of $\angle{bac}$.
Draw a ray starting at $a$ and passing through $t$.
Let $d$ be a point on ray $at$ such that $d$ is between $a$ and $t$.
Then there is some line $x y$ passing through $t$, where $x$ is a point on one side of angle $bac$ and $y$ is a point on the other.
Second Form
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Source of Name
This entry was named for Euclid.
Also see
- Euclid's Fifth Postulate, an analogue of Euclid's Axiom in the context of Second-Order Logic.
Sources
- June 1999: Alfred Tarski and Steven Givant: Tarski's System of Geometry (The Bulletin of Symbolic Logic Vol. 5, no. 2: 175 – 214) : Page 183 : Axiom $10$
Illustration courtesy of Steven Givant.
