For every partition of all the points on a line into two nonempty sets such that no point of either lies between two points of the other, there is a point of one set which lies between every other point of that set and every point of the other set.
Dedekind's Axiom
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Cite this as:
Weisstein, Eric W. "Dedekind's Axiom." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/DedekindsAxiom.html