On The Puzzling History of Euclid’s Fifth Postulate

At the outset of Euclid’s Elements he offers twenty-three definitions, five postulates, and five common notions (sometimes translated as “axioms”). Of the five postulates, the fifth is the most troubling. It is known as the Parallel Postulate. The word postulate can be roughly translated to mean “request,” “question,” or “hypothesis” (postulat in Latin means “asked”).

The Parallel Postulate is translated from Greek as follows:

“That, if a straight line falling on two straight lines make the interior angles on the same side less than two right angles, the two straight lines, if produced indefinitely, meet on that side on which are the angles less than the two right angles.”

In the picture above, two lines are intersected and their inner angles are less than two right angles, and therefore both lines will meet if extended indefinitely. It defines parallelism as contrasted with It also deals with concepts of divergence and convergence -the implication is a certain degree of motion.

Why does the Parallel Postulate not occur as a Proposition in Euclid’s Elements? Why is it not, for example, a demonstrated reductio? In some ways the Parallel Postulate begs to have a proof of its claims (note: Euclid’s Proposition 27 in Book I).

The problem with the Parallel Postulate is that a “proof” or demonstration has not been sufficiently made by means of reason and thus requires a visual demonstration to understand its claim. The Parallel Postulate requires significant use of the human imagination. We imagine someone drawing two straight and parallel lines and then another line ‘falling’ on the two unparallel lines at an angle that is not perpendicular in a way that both lines will eventually meet. The key term in the Parallel Postulate is indefinitely. In all likelihood, Euclid used the term indefinitely to encourage us to consider a hypothetical exercise in drawing two parallel lines onward without actually doing so -does Euclid’s surface also continue indefinitely? And if so, is the surface indefinitely flat? The crucial distinguishing factor is that Euclid invites us to draw these two parallel lines on a what we might call a mental plane, perhaps not even an existing surface.

Many mathematicians have attempted to prove the Parallel Postulate but to little avail. Ptolemy thought he had proved the Parallel Postulate but Proclus found an error in his proof, and Proclus could not prove the Parallel Postulate either. However, mathematicians have also explored what might happen if the Parallel Postulate was untrue: names like Ibn Al-Haytham, the great poet-mathematician Omar Khayyam, Nasir al-din Al-Tusi, the famous polymath Muhammad ibn Musa al-Khwarizmi, Giovanni Saccheri, Adelard, Descartes, Janos Bolyai, Newton, Leibniz, Carl Gauss, and Nikolai Lobachevsky. The modern shift from the Middle Ages onward was an exploration into the possibilities of geometry. They found that the negation of Euclid’s Parallel Postulate gives rise to new forms of geometry (non-Euclidean geometries). The key distinction relies on the surface upon which the parallel lines are constructed -is it indefinitely flat or is there a curvature to the surface, as is the case with objects in the physical world. In the real world, elliptical geometry better describes shapes that have being, while other forms of geometry are orderly but puzzling, like M.C. Escher’s artwork which displays hyperbolic geometry. Einstein used non-Euclidean geometry to describe the ways in which the space-time continuum becomes warped in the presence of matter in his General Theory of Relativity. This means that the curvature of space implies that straight and parallel lines will, in fact, meet at one point if extended indefinitely.

Thus what began as an ancient quest in search of geometric and Platonic perfection (Pythagoras, Euclid, Proclus etc) was transformed into a project to better understand and map the world around us (Descartes, Newton, Lobachevsky, Einstein etc). The investigation of the true “earth measurement” continues. Defenders of Euclidian geometry argue that Euclid never intended for his Elements to resemble anything existing in the world around us. They say his geometry is pure abstraction in search of perfection. However this poses certain problems because Euclid’s geometry is not pure abstraction. It relies upon a certain understanding and demonstration of postulates in this world -i.e. not in some fabled celestial or divine realm. Therefore the question of whether indefinitely parallel lines will ever meet remains a vexing theoretical quandary, and indeed it is worth entertaining the modern position of doubting Euclid and exploring where new geometries take us.


For this reading I used the wonderful translation of Euclid’s Elements by Thomas L. Heath for Green Lion Press. Mr. Heath was a Cambridge scholar who translated Euclid directly from the original Greek in the early 20th century.

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