Commentary on the First Book of Euclid's Elements

(below 354, 20) book III (IV) begins; on folio 233 (below 433) book IV begins, but in reality, it is a scholion by some uncertain person. On the contrary, on folio 109 (below 198), where even by Barocius’s own judgment the 'End of the Principles' and the beginning of the propositions is found, there is not the slightest sign of a new matter. When I saw these things, I no longer doubted that the partition of the commentaries into four books originated from a copyist or scribe, whose copy was the source of all the copies we know. Barocius divided those individual books into chapters and commentaries, whom Taylor followed, and whom I would have followed had I not thought that the genuine form should be restored to the work of Proclus as much as possible. It seems to me that Proclus continued his commentaries in such a way that he would insert only the words of Euclid, while separating his own, which were arranged most clearly, with no larger interval. Furthermore, it is not at all certain by what right Proclus's work received the name of commentaries; you will say it is much more likely from the end of the first prologue (below 47, 7) that Proclus was going to write a study of mathematical discipline. But you will extract nothing certain from that. Therefore, I have retained the title handed down for three centuries, added only six Latin inscriptions, and given the definitions, postulates, axioms, and propositions of Euclid their own numbers, to make the survey of the entire work easier. To make individual items easier to find, I have added two indices, one of names, the other of subjects and words. Finally, the page numbers of both Grynaeus and Barocius are placed in the upper part of each page, so that a longer delay does not hinder the reader looking for cited passages. The more people who are pleased to have Proclus restored in Greek, the less I will regret the not-insignificant labor consumed in it.

At the Curia by the Saale, in the month of January of the year 1873.

Godofredus Friedlein.