Euclid, Greek mathematician of the 3rd century BC; we are ignorant not only of the dates of his birth and death, but also of his parentage, his teachers, and the residence of his early years. In some of the editions of his works he is called Megarensis, as if he had been born at Megara in Greece, a mistake which arose from confounding him with Euclid of Megara, a disciple of Socrates. Proclus Diadochus (AD 412-485), the authority for most of our information regarding Euclid, states in his commentary on the first book of the Elements that Euclid lived in the time of Ptolemy I, king of Egypt, who reigned from 323-285 BC, that he was younger than the associates of Plato, but older than Eratosthenes (276-196 BC) and Archimedes (287-212 BC). Euclid is said to have founded the mathematical school of Alexandria, which was at the time becoming a center not only of commerce, but also of learning and research, and for this service to the cause of exact science he would have deserved commemoration, even if his writings had not secured him a worthier title to fame. Proclus preserves a reply made by Euclid to King Ptolemy, who asked whether he could not learn geometry more easily than by studying the Elements - "There is no royal road to geometry." Pappus of Alexandria, in his Mathematical Collection, says that Euclid was a man of mild and inoffensive temperament, unpretending, and kind to all genuine students of mathematics. This being all that is known of the life and character of Euclid, it only remains therefore to speak of his works.

Among those which have come down to us the most remarkable is the Elements (*Stoicheia*). They consist of thirteen books; two more are frequently added, but there is reason to believe that they are the work of a later mathematician, Hypsciles of Alexandria.

The question has often been mooted, to what extent Euclid, in his Elements, is a discoverer or a compiler. To this question no entirely satisfactory answer can be given, for scarcely any of the writings of the earlier geometers have come down to our times. We are mainly dependent on Pappus and Proclus for the scanty notices we have of Euclid’s predecessors, and of the problems which engaged their attention; for the solution of problems, and not the discovery of theorems, would seem to have been their principal object. From these authors we learn that the property of the right angled triangle has been found out, the principles of geometrical analysis laid down, the restriction of constructions in plane geometry to the straight line and the circle agreed upon, the doctrine of proportion, for both commensurables and incommensurables, as well as loci, plane and solid, and some of the properties of the conic sections investigated, the five regular solids (often called the Platonic bodies) and the relation between the volume of a cone or pyramid and that of its circumscribed cylinder or prism discovered. Elementary works had been written, and the famous problem of the duplication of the cube reduced to the determination of two mean proportionals between two given straight lines. Notwithstanding this amount of discovery, and all that it implied, Euclid must have made a great advance beyond his predecessors (we are told that "he arranged the discoveries of Eudoxus, perfected those of Theaetetus, and reduced to invincible demonstration many things which had previously been more loosely proved"), for his Elements supplanted all similar treatises, and, as Apollonius received the title of "the great geometer," so Euclid has come down to latter ages as "the elementator."

For the past twenty centuries parts of the Elements, notably the first six books, have been used as an introduction to geometry. Though they are now to some extent superseded in most countries, their long retention is a proof that they were, at any rate, no unsuitable for such a purpose. They are, speaking generally, not too difficult for novices in the science; the demonstrations are rigorous, ingenious and often elegant the mixture of problems and theorems gives perhaps some variety, and makes their study less monotonous; and, if regard be had merely to the metrical properties of space as distinguished from the graphical, hardly any cardinal geometrical truths are omitted. With these excellence are combined a good many defects, some of them inevitable to a system based on a very few axioms and postulates. Thus the arrangement of the propositions seems arbitrary; associated theorems and problems are not grouped together; the classification, in short, is imperfect. Other objections, not to mention minor blemishes, are the prolixinty of the style, arising partly from a defective nomenclature, the treatment of parallels depending on an axiom which is not axiomatic, and the sparing use of superposition as a method of proof.

Of the 33 ancient books subservient to geometrical analysis, Pappus enumerates first the Data (*Dedomena*), of Euclid. He says it contained 90 propositions, the scope of which he describes; it now contains 95. It is not easy to explain this discrepancy, unless we suppose that some of the propositions, as they existed in the time of Pappus, have since been split into two, or that what were once scholia have since been erected into propositions. The object of the Data is to show that when certain things – lines, angles, spaces, ratios, etc – are given by hypothesis, certain other things are given, that is, determinable. The book, as we are expressly told, and as we may gather from its contents, was intended for the investigation of problems; and it has been conjectured that Euclid must have extended the method of the Data to the investigation of theorems. What prompts this conjecture is the similarity between the analysis of a theorem and the method, common enough in the Elements, of *reductio ad absurdum* – the one setting out from the supposition that the theorem is true, the other from the supposition that it is false, thence in both cases deducing a chain of consequences that end in a conclusion previously known to be true or false.

The Introduction to Harmony (*Eisagoge armonike*), and the Section of the Scale (*Katatome kanonos*), treat of music. There is good reason for believing that one at any rate, and probably both of these books are not by Euclid. No mention is made of them by any writer prior to Ptolemy (AD 140), or by Ptolemy himself, and in no ancient codex are they ascribed to Euclid.

The Phaenomena (*Phainomena*) contains an exposition of the appearances produced by the motion attributed to the celestial sphere. Pappus, in a few remarks prefatory to his sixth book complains of the faults, both of omission and commission, of writers of astronomy, and cites as an example of the former the second theorem of Euclid’s Phaenomena, whence, and from the interpolation of other proofs, David Gregory infers that this treatise is corrupt.

The Optics and Catoptrics (*Optika, Katoptrika*) are ascribed to Euclid by Proclus, and by Marinus in his preface to the Data, but no mention is made of them by Pappus. This latter circumstance, taken in connection with the fact that two of the propositions in the sixth book of the Mathematical Collection prove the same things as three in the Optics, is one of the reasons given by Gregory for deeming the work spurious. Several other reasons will be found in Gregory’s preface to his edition of Euclid’s works.

In some editions of Euclid’s works there is given a book on the Divisions of Superfices, which consists of a few propositions, showing how a straight line may be drawn to divide in a given ratio triangles, quadrilaterals and pentagons. This was supposed by John Dee of London, who transcribed or translated it, and entrusted it for publication to his friend Federico Commandino of Urbino, to be the treatise of Euclid referred to by Proclus as to *peri diaireseon biblion*. Dee mentions that, in the copy from which he wrote, the book was ascribed to Machomet of Baghdad, and adduces two or three reasons for thinking it to be Euclid’s. This opinion, however, he does not seem to have held very strongly, nor does it appear that it was adopted by Commandino. The book does not exist in Greek.

The fragment, in Latin, De levi et ponderoso, which is of no value, and was printed at the end of Gregory’s edition only in order that nothing might be left out, is mentioned neither by Pappus nor Proclus, and occurs first in Bartholomew Zamberti’s edition of 1537. There is no reason for supposing it to be genuine.

The following works attributed to Euclid are not now extant -

1. Three books on Porisms (*Peri ton porismaton*) are mentioned both by Pappus and Proclus, and the former gives an abstract of them, with the lemmas assumed.

2. Two books are mentioned, named Topon pros epithaneia, which is rendered Locorum ad superficiem by Commandino and subsequent geomters. These books were subservient to the analysis of loci, but the four lemmas which refer to them and which occur at the end of the seventh book of the Mathematical Collection, throw very little light on their contents. R. Simson’s opinion was that they treated of curves of double curvature, and he intended at one time to write a treatise on the subject.

3. Pappus says that Euclid wrote four books on the Conic Sections (*biblia tessara Konikon*), which Apollonius of Perga amplified, and to which he added four more. It is known that, in the time of Euclid, the parabola was considered as the section of a right angled cone, the ellipse that of an acute acute angled cone, the hyperbola that of an obtuse angled cone, and that Appolonius was the first to show that the three sections could be obtained from any cone. There is good ground therefore for supposing that the first four books of Apollonius’s Conics, which are still extant, resemble Euclid’s Conics even less than Euclid’s Elements do those of Eudoxus and Theaetetus.

4. A book on Fallacies (*Peri pheudarion*) is mentioned by Proclus, who says Euclid wrote it for the purpose of exercising beginners in the detection of errors in reasoning.

From the eleventh edition of The Encyclopedia, 1911. Public domain. Some editing has been done for clarity and for other reasons.