| A part of a straight line cannot be in the plane of reference and a part in plane more elevated. | |
| For, if possible, let a part AB of the straight line ABC be in the plane of reference, and a part BC be in a plane more elevated. | |
| Then there is in the plane of reference some straight line continuous with AB in a straight line. Let it be BD. Therefore AB is a common segment of the two straight lines ABC and ABD, which is impossible, since, if we describe a circle with center B and radius AB, then the diameters cut off unequal circumferences of the circle. | |
| Therefore, a part of a straight line cannot be in the plane of reference and a part in plane more elevated. | |
| Q. E. D. | |
