# Isaac Todhunter: *Euclid* Introduction

In 1905 the London publisher Joseph Malaby Dent had the idea of producing the Everyman's Library, a cheap series of reprints of classical texts. His original aim was to produce 1000 volumes, creating a library of world literature, which would sell at one shilling per volume. Dent wanted the series:-

Below we reproduce Todhunter's Introductory Remarks to his edition of Euclid written in October 1862.

... to appeal to every kind of reader: the worker, the student, the cultured man, the child, the man and the woman [so that] for a few shillings the reader may have a whole bookshelf of the immortals; for five pounds (which will procure him with a hundred volumes) a man may be intellectually rich for life.In 1932 Euclid's

*Elements*was published as No 891 in the Everyman's Library. Todhunter's*Euclid,*published in 1962, was chosen to be the edition reprinted and an Introduction to the Everyman's Library book was written by Sir Thomas L Heath.Below we reproduce Todhunter's Introductory Remarks to his edition of Euclid written in October 1862.

### INTRODUCTORY REMARKS

The subject of Plane Geometry is here presented to the student arranged in six books, and each book is subdivided into propositions. The propositions are of two kinds,

*problems*and

*theorems.*In a problem something is required to be done; in a theorem some new principle is asserted to be true.

A proposition consists of various parts. We have first the general enunciation of the problem or theorem; as for example,

*To describe an equilateral triangle on a given finite straight line,*or

*Any two angles of a triangle are together less than two right angles.*After the general enunciation follows the discussion of the proposition. First, the enunciation is repeated and applied to the particular figure which is to be considered; as for example,

*Let A B be the given straight line: it is required to describe an equilateral triangle on AB.*The

*construction*then usually follows, which states the necessary straight lines and circles which must be drawn in order to constitute the solution of the problem, or to furnish assistance in the demonstration of the theorem. Lastly, we have the demonstration itself, which shows that the problem has been solved, or that the theorem, is true.

Sometimes, however, no construction is required;. and sometimes the construction and demonstration are combined.

The demonstration is a process of reasoning in which we draw inferences from results already obtained. These results consist partly of truths established in former propositions, or admitted as obvious in commencing the subject, and partly of truths which follow from the

*construction*that has been made, or which are given in the

*supposition*of the proposition itself. The word

*hypothesis*is used in the same sense as

*supposition.*

To assist the student in following the steps of the reasoning, references are given to the results already obtained which are required in the demonstration. Thus I. 5 indicates that we appeal to the result established in the fifth proposition of the First Book;

*Constr.*is sometimes used as an abbreviation of

*Construction,*and

*Hyp.*as an abbreviation of

*Hypothesis.*

It is usual to place the letters Q.E.F. at the end of the discussion of a problem, and the letters Q.E.D. at the end of the discussion of a theorem. Q.E.F. is an abbreviation for

*quod erat faciendum,*that is, which was to be done; and Q.E.D. is an abbreviation for

*quod erat demonstrandum,*that is, which was to be proved.

I Todhunter

1862

Last Updated August 2007