Question

One of Euclid’s postulates has always seemed to be too complex, and Euclid himself thought it should be proved as a theorem--but it cannot be. Which postulate is this

Answer 1

Postulate 5, The Parallel postulate: If a straight line intersects two other straight lines, and so makes the two interior angles on one side of it together less than two right angles, then the other straight lines will meet at a point if extended far enough on the side on which the angles are less than two right angles.

The parallel postulate is needed to derive the elements, but it seems too complex.