Euclid, Elements
Thomas L. Heath, Sir Thomas Little Heath, Ed.

objection raised to a particular proposition, he uses such expressions as “perhaps someone may object” (ἴσως δ̓ ἄν τινες ἐνσταῖεν...): for sometimes other words in the same passage indicate that the objection had actually been taken by someone¹. Speaking generally, we shall not be justified in concluding that Proclus is stating something new of his own unless he indicates this himself in express terms.

As regards the form of Proclus' references to others by name, van Pesch notes that he very seldom mentions the particular work from which he is borrowing. If we leave out of account the references to Plato's dialogues, there are only the following references to books: the Bacchae of Philolaus², the Symmikta of Porphyry³, Archimedes On the Sphere and Cylinder⁴, Apollonius On the cochlias⁵, a book by Eudemus on The Angle⁶, a whole book of Posidonius directed against Zeno of the Epicurean sect⁷, Carpus' Astronomy⁸, Eudemus' History of Geometry⁹, and a tract by Ptolemy on the parallel-postulate¹⁰.

Again, Proclus does not always indicate that he is quoting something at second-hand. He often does so, e.g. he quotes Heron as the authority for a statement about Philippus, Eudemus as attributing a certain theorem to Oenopides etc.; but he says on 1. 12 that “Oenopides first investigated this problem, thinking it useful for astronomy” when he cannot have had Oenopides' work before him.

It has been said above that Proclus was in the habit of stating in his own words the substance of the things which he borrowed. We are prepared for this when we find him stating that he will select the best things from ancient commentaries and “cut short their interminable diffuseness,” that he will “briefly describe” (συντομως ἱστορῆσαι) the other proofs of 1. 20 given by Heron and Porphyry and also the proofs of 1. 25 by Menelaus and Heron. But the best evidence is of course to be found in the passages where he quotes works still extant, e.g. those of Plato, Aristotle and Plotinus. Examination of these passages shows great divergences from the original; even where he purports to quote textually, using the expressions “Plato says,” or “Plotinus says,” he by no means quotes word for word¹¹. In fact, he seems to have had a positive distaste for quoting textually from other works. He cannot conquer this even when quoting from Euclid; he says in his note on 1. 22, “we will follow the words of the geometer” but fails, nevertheless, to reproduce the text of Euclid unchanged¹².

We now come to the sources themselves from which Proclus drew