Euclid’s “Elements” is a mathematical and geometric treatise comprising about 500 pages and consisting of 13 books written by the ancient Greek mathematician Euclid in Alexandria ca. 300 BC. It is a collection of definitions, postulates (axioms), common notions (unproved lemmata), propositions and lemmata (i.e. theorems and constructions), corollaries (for which in some editions the Greek word “porisms” is used) and mathematical proofs of the propositions. The 13 books cover geometry, now known as Euclidean, and the ancient Greek version of elementary number theory. The work also includes an algebraic system that has become known as algebraic geometry, which is powerful enough to solve many algebraic problems, including the problem of finding the square root of a number.
The “Elements” are still considered a masterpiece in the application of logic in mathematics. In historical context, the work has proven enormously influential in many areas of science. Scientists like Nicolaus Copernicus, Johannes Kepler, Galileo Galilei, and Sir Isaac Newton were all influenced by the “Elements”, and applied their knowledge of this work to their work. Mathematicians and philosophers, such as Bertrand Russell, Alfred North Whitehead, and Baruch Spinoza, have attempted to create their own foundational “Elements” for their respective disciplines by adopting the axiomatized deductive structures that Euclid’s work introduced.
Over the years, the “Elements” have been copied, recopied, modified, commented upon and interpreted unceasingly. Only the painstaking comparison of all available sources allowed Heiberg in 1888 to essentially reconstruct the original version. The most important source (M.S. 190 ; this manuscript dates from the 10th century) was discovered in the treasury of the Vatican when Napoleon’s troops invaded Rome in 1809. Heiberg’s text has been translated into all scientific languages.
An important English translation is a translation by Sir Thomas L.Heath2775 in 1908 (second enlarged edition 1926). However, for a modern reader, it is hard to read because it uses an archaic state of the English language. A more modern English translation6419 was done by Prof. Richard Fitzpatrick (University of Texas at Austin) in 2007, and other considerable efforts to use a more modern mathematical language for the “Elements” have been made by other authors like Daniel Callahan626, or the authors of proofwiki, who, for instance, contributed titles to the propositions and definitions, missing in the original version. Those titles have also been used in the following edition.
A unique feature of this online edition of the “Elements” BookofProofs is that it combines the modern English translation1 of the original text6419 with supplemental modern versions of the theorems and definitions. We hope that this unique feature of this online edition will help the reader to better understand and appreciate the timeless achievements of Euclid’s original work.
1 Please note that Prof. Fitzpatrick’s original translation6419 is not (!) licensed under “CC BY-SA 3.0”, however, extracts from this translation have been compiled in this online edition which is (!) made available under the “CC BY-SA 3.0” license with the kind permission of Prof. Richard Fitzpatrick.
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 Callahan, Daniel: “Euclid’s ‘Elements’ Redux”, http://starrhorse.com/euclid/, 2014
 Fitzpatrick, Richard: “Euclid’s Elements of Geometry”, http://farside.ph.utexas.edu/Books/Euclid/Euclid.html, 2007
 Heiberg, I.L, Menge H: “Euclidis Opera Omnia”, https://archive.org/details/euclidisoperaomn01eucluoft, 1883
 Health, T.L.: “The Thirteen Books of Euclid’s Elements – With Introduction and Commentary by T. L. Health”, Cambridge at the University Press, 1968, Vol 1, 2, 3