Proof: By Euclid
(related to Proposition: Prop. 11.01: Straight Line cannot be in Two Planes)
∎
- For, if possible, let some part, AB, of the straight line ABC be in a reference plane, and some part, BC, in a more elevated (plane).
- In the reference plane, there will be some straight line continuous with, and straight-on to, AB.
- Let it be BD.
- Thus, AB is a common segment of the two (different) straight lines ABC and ABD.
- The very thing is impossible, inasmuch as if we draw a circle with center B and radius AB then the diameters (ABD and ABC) will cut off unequal circumferences of the circle.

- Thus, some part of a straight line cannot be in a reference plane, and (some part) in a more elevated (plane).
- (Which is) the very thing it was required to show.
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References
Adapted from (subject to copyright, with kind permission)
- Fitzpatrick, Richard: Euclid's "Elements of Geometry"
Footnotes