The pre-Euclidean period

The Greeks divided the field of mathematics into arithmetic (the study of “multitude,” or discrete quantity) and geometry (that of “magnitude,” or continuous quantity) and considered both to have originated in practical activities. Proclus, in his Commentary on Euclid, observes that geometry—literally, “measurement of land”—first arose in surveying practices among the ancient Egyptians, for the flooding of the Nile compelled them each year to redefine the boundaries of properties. Similarly, arithmetic started with the commerce and trade of Phoenician merchants. Although Proclus wrote quite late in the ancient period (in the 5th century ce), his account drew upon views proposed much earlier—by Herodotus (mid-5th century bce), for example, and by Eudemus, a disciple of Aristotle (late 4th century bce).

However plausible, this view is difficult to check, for there is only meagre evidence of practical mathematics from the early Greek period (roughly, the 8th through the 4th century bce). Inscriptions on stone, for example, reveal use of a numeral system the same in principle as the familiar Roman numerals. Herodotus seems to have known of the abacus as an aid for computation by both Greeks and Egyptians, and about a dozen stone specimens of Greek abaci survive from the 5th and 4th centuries bce. In the surveying of new cities in the Greek colonies of the 6th and 5th centuries, there was regular use of a standard length of 70 plethra (one plethron equals 100 feet) as the diagonal of a square of side 50 plethra; in fact, the actual diagonal of the square is 50Square root of√2 plethra, so this was equivalent to using 7/5 (or 1.4) as an estimate for Square root of√2, which is now known to equal 1.414…. In the 6th century bce the engineer Eupalinus of Megara directed an aqueduct through a mountain on the island of Samos, and historians still debate how he did it. In a further indication of the practical aspects of early Greek mathematics, Plato describes in his Laws how the Egyptians drilled their children in practical problems in arithmetic and geometry; he clearly considered this a model for the Greeks to imitate.

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Such hints about the nature of early Greek practical mathematics are confirmed in later sources—for example, in the arithmetic problems in papyrus texts from Ptolemaic Egypt (from the 3rd century bce onward) and the geometric manuals by Heron of Alexandria (1st century ce). In its basic manner this Greek tradition was much like the earlier traditions in Egypt and Mesopotamia. Indeed, it is likely that the Greeks borrowed from such older sources to some extent.

What was distinctive of the Greeks’ contribution to mathematics—and what in effect made them the creators of “mathematics,” as the term is usually understood—was its development as a theoretical discipline. This means two things: mathematical statements are general, and they are confirmed by proof. For example, the Mesopotamians had procedures for finding whole numbers a, b, and c for which a² + b² = c² (e.g., 3, 4, 5; 5, 12, 13; or 119, 120, 169). From the Greeks came a proof of a general rule for finding all such sets of numbers (now called Pythagorean triples): if one takes two whole numbers p and q, both being even or both odd and such that pq is a square number, then a = (p − q)/2, b = Square root of√pq, and c = (p+ q)/2. As Euclid proves in Book X of the Elements, numbers of this form satisfy the relation for Pythagorean triples. Further, the Mesopotamians appear to have understood that sets of such numbers a, b, and c form the sides of right triangles, but the Greeks proved this result (Euclid, in fact, proves it twice: in Elements, Book I, proposition 47, and in a more general form in Elements, Book VI, proposition 31), and these proofs occur in the context of a systematic presentation of the properties of plane geometric figures.

The Elements, composed by Euclid of Alexandria about 300 bce, was the pivotal contribution to theoretical geometry, but the transition from practical to theoretical mathematics had occurred much earlier, sometime in the 5th century bce. Initiated by men like Pythagoras of Samos (late 6th century) and Hippocrates of Chios (late 5th century), the theoretical form of geometry was advanced by others, most prominently the Pythagorean Archytas of Tarentum, Theaetetus of Athens, and Eudoxus of Cnidus (4th century). Because the actual writings of these men do not survive, knowledge about their work depends on remarks made by later writers. While even this limited evidence reveals how heavily Euclid depended on them, it does not set out clearly the motives behind their studies.

It is thus a matter of debate how and why this theoretical transition took place. A frequently cited factor is the discovery of irrational numbers. The early Pythagoreans held that “all things are number.” This might be taken to mean that any geometric measure can be associated with some number (that is, some whole number or fraction; in modern terminology, rational number), for in Greek usage the term for number, arithmos, refers exclusively to whole numbers or, in some contexts, to ordinary fractions. This assumption is common enough in practice, as when the length of a given line is said to be so many feet plus a fractional part. However, it breaks down for the lines that form the side and diagonal of the square. (For example, if it is supposed that the ratio between the side and diagonal may be expressed as the ratio of two whole numbers, it can be shown that both of these numbers must be even. This is impossible, since every fraction may be expressed as a ratio of two whole numbers having no common factors.) Geometrically, this means that there is no length that could serve as a unit of measure of both the side and diagonal; that is, the side and diagonal cannot each equal the same length multiplied by (different) whole numbers. Accordingly, the Greeks called such pairs of lengths “incommensurable.” (In modern terminology, unlike that of the Greeks, the term “number” is applied to such quantities as Square root of√2, but they are called irrational.)

This result was already well known at the time of Plato and may well have been discovered within the school of Pythagoras in the 5th century bce, as some late authorities like Pappus of Alexandria (4th century ce) maintain. In any case, by 400 bce it was known that lines corresponding to Square root of√3, Square root of√5, and other square roots are incommensurable with a fixed unit length. The more general result, the geometric equivalent of the theorem that Square root of√p is irrational whenever p is not a rational square number, is associated with Plato’s friend Theaetetus. Both Theaetetus and Eudoxus contributed to the further study of irrationals, and their followers collected the results into a substantial theory, as represented by the 115 propositions of Book X of the Elements.

The discovery of irrationals must have affected the very nature of early mathematical research, for it made clear that arithmetic was insufficient for the purposes of geometry, despite the assumptions made in practical work. Further, once such seemingly obvious assumptions as the commensurability of all lines turned out to be in fact false, then in principle all mathematical assumptions were rendered suspect. At the least it became necessary to justify carefully all claims made about mathematics. Even more basically, it became necessary to establish what a reasoning has to be like to qualify as a proof. Apparently, Hippocrates of Chios, in the 5th century bce, and others soon after him had already begun the work of organizing geometric results into a systematic form in textbooks called “elements” (meaning “fundamental results” of geometry). These were to serve as sources for Euclid in his comprehensive textbook a century later.

The early mathematicians were not an isolated group but part of a larger, intensely competitive intellectual environment of pre-Socratic thinkers in Ionia and Italy, as well as Sophists at Athens. By insisting that only permanent things could have real existence, the philosopher Parmenides (5th century bce) called into question the most basic claims about knowledge itself. In contrast, Heracleitus (c. 500 bce) maintained that all permanence is an illusion, for the things that are perceived arise through a subtle balance of opposing tensions. What is meant by “knowledge” and “proof” thus came into debate.

Mathematical issues were often drawn into these debates. For some, like the Pythagoreans (and, later, Plato), the certainty of mathematics was held as a model for reasoning in other areas, like politics and ethics. But for others mathematics seemed prone to contradiction. Zeno of Elea (5th century bce) posed paradoxes about quantity and motion. In one such paradox it is assumed that a line can be bisected again and again without limit; if the division ultimately results in a set of points of zero length, then even infinitely many of them sum up only to zero, but, if it results in tiny line segments, then their sum will be infinite. In effect, the length of the given line must be both zero and infinite. In the 5th century bce a solution of such paradoxes was attempted by Democritus and the atomists, philosophers who held that all material bodies are ultimately made up of invisibly small “atoms” (the Greek word atomon means “indivisible”). But in geometry such a view came into conflict with the existence of incommensurable lines, since the atoms would become the measuring units of all lines, even incommensurable ones. Democritus and the Sophist Protagoras puzzled over whether the tangent to a circle meets it at a point or a line. The Sophists Antiphon and Bryson (both 5th century bce) considered how to compare the circle to polygons inscribed in it.

The pre-Socratics thus revealed difficulties in specific assumptions about the infinitely many and the infinitely small and about the relation of geometry to physical reality, as well as in more general conceptions like “existence” and “proof.” Philosophical questions such as these need not have affected the technical researches of mathematicians, but they did make them aware of difficulties that could bear on fundamental matters and so made them the more cautious in defining their subject matter.

Any such review of the possible effects of factors such as these is purely conjectural, since the sources are fragmentary and never make explicit how the mathematicians responded to the issues that were raised. But it is the particular concern over fundamental assumptions and proofs that distinguishes Greek mathematics from the earlier traditions. Plausible factors behind this concern can be identified in the special circumstances of the early Greek tradition—its technical discoveries and its cultural environment—even if it is not possible to describe in detail how these changes took place.

The Elements

The principal source for reconstructing pre-Euclidean mathematics is Euclid’s Elements, for the major part of its contents can be traced back to research from the 4th century bce and in some cases even earlier. The first four books present constructions and proofs of plane geometric figures: Book I deals with the congruence of triangles, the properties of parallel lines, and the area relations of triangles and parallelograms; Book II establishes equalities relating to squares, rectangles, and triangles; Book III covers basic properties of circles; and Book IV sets out constructions of polygons in circles. Much of the content of Books I–III was already familiar to Hippocrates, and the material of Book IV can be associated with the Pythagoreans, so that this portion of the Elements has roots in 5th-century research. It is known, however, that questions about parallels were debated in Aristotle’s school (c. 350 bce), and so it may be assumed that efforts to prove results—such as the theorem stating that for any given line and given point, there always exists a unique line through that point and parallel to the line—were tried and failed. Thus, the decision to found the theory of parallels on a postulate, as in Book I of the Elements, must have been a relatively recent development in Euclid’s time. (The postulate would later become the subject of much study, and in modern times it led to the discovery of the so-called non-Euclidean geometries.)

Book V sets out a general theory of proportion—that is, a theory that does not require any restriction to commensurable magnitudes. This general theory derives from Eudoxus. On the basis of the theory, Book VI describes the properties of similar plane rectilinear figures and so generalizes the congruence theory of Book I. It appears that the technique of similar figures was already known in the 5th century bce, even though a fully valid justification could not have been given before Eudoxus worked out his theory of proportion.

Books VII–IX deal with what the Greeks called “arithmetic,” the theory of whole numbers. It includes the properties of numerical proportions, greatest common divisors, least common multiples, and relative primes (Book VII); propositions on numerical progressions and square and cube numbers (Book VIII); and special results, like unique factorization into primes, the existence of an unlimited number of primes, and the formation of “perfect numbers”—that is, those numbers that equal the sum of their proper divisors (Book IX). In some form Book VII stems from Theaetetus and Book VIII from Archytas.

Book X presents a theory of irrational lines and derives from the work of Theaetetus and Eudoxus. The remaining books treat the geometry of solids. Book XI sets out results on solid figures analogous to those for planes in Books I and VI; Book XII proves theorems on the ratios of circles, the ratios of spheres, and the volumes of pyramids and cones; Book XIII shows how to inscribe the five regular solids—known as the Platonic solids—in a given sphere (compare the constructions of plane figures in Book IV). The measurement of curved figures in Book XII is inferred from that of rectilinear figures; for a particular curved figure, a sequence of rectilinear figures is considered in which succeeding figures in the sequence become continually closer to the curved figure; the particular method used by Euclid derives from Eudoxus. The solid constructions in Book XIII derive from Theaetetus.

In sum the Elements gathered together the whole field of elementary geometry and arithmetic that had developed in the two centuries before Euclid. Doubtless, Euclid must be credited with particular aspects of this work, certainly with its editing as a comprehensive whole. But it is not possible to identify for certain even a single one of its results as having been his discovery. Other, more advanced fields, though not touched on in the Elements, were already being vigorously studied in Euclid’s time, in some cases by Euclid himself. For these fields his textbook, true to its name, provides the appropriate “elementary” introduction.

One such field is the study of geometric constructions. Euclid, like geometers in the generation before him, divided mathematical propositions into two kinds: “theorems” and “problems.” A theorem makes the claim that all terms of a certain description have a specified property; a problem seeks the construction of a term that is to have a specified property. In the Elements all the problems are constructible on the basis of three stated postulates: that a line can be constructed by joining two given points, that a given line segment can be extended in a line indefinitely, and that a circle can be constructed with a given point as centre and a given line segment as radius. These postulates in effect restricted the constructions to the use of the so-called Euclidean tools—i.e., a compass and a straightedge or unmarked ruler.