What is the relationship between Pasch's axiom, plane separation postulate, and betweeness?
Specifically, are any of the following derivable from each other:
- Pasch's axiom
- In the plane, a line partitions its complement into two sets, $P$ and $Q$, such that any segment with points in both $P$ and $Q$ must intersect the line
- Similar to #2, but adding and any segment in with both endpoints in $P$ does not intersect the line
- Similar to #2 or #3, but using circle or triangle instead of line.
Wikipedia claims an equivalence between Pasch's Axiom and Plane Separation Axiom but does not prove it or even provide a definition of the Plane Separation Axiom they claim is equivalent.
Similarly, it's been argued that betweeness is undefined without Pasch's axiom. I have trouble accepting this for several reasons:
- Betweeness seems to me to come directly from Euclid's Common Notions, which implicitly assume a. a total ordering over segments of the same line and b. a notion of part-whole decomposition of a segment. It seems clear that these can be used to define betweenesss.
- Hilbert's axioms define betweeness without using Pasch's axiom (which is incorporated separately); likewise for Tarski
- The concept of betweeness (or its violation) does not seem to depend on plane separation, and vice versa. I can imagine models which violate one of them but not both.
Update: Which Axioms?
My preferred system of axioms is that of Tarski, as eventually simplified, e.g. see here. I'm particularly interested in what happens if the Axiom of Continuity is replaced with an axiom only asserting the points needed for an Euclidean Field (but not full completeness).