The most important references are those by Diogenes Laërtius, who wrote short biographies of many early philosophers, and the 10th century CE Souda, a catalog of several thousand persons and terms from antiquity.
[1]And in another passage, Philip is listed among the members of the Academy:His [Plato's] disciples were Speusippus of Athens, Xenocrates of Chalcedon, Aristotle of Stagira, Philippus of Opus...[2]In the Souda, Philip is listed anonymously under the heading of philosophos ("philosopher"), his name being lost from the beginning of the entry:Philosopher who divided the Laws of Plato into 12 books; for he himself is said to have added the 13th.
It was not until the early 18th century that Ludolf Küster, who edited the Souda, identified this anonymous entry with the Philip of Opus mentioned by Diogenes Laërtius.
Eudoxus, Endæmon, Callippus, Melo, Philip, Hipparchus, Aratus, and others, following in the steps of the preceding, found, by the use of instruments, the rising and setting of the stars and the changes of the seasons, and left treatises thereon for the use of posterity.
[5]Pliny the Elder, in Natural History (1st Century CE):It is a remarkable fact, and rarely the case, that Philippus, Callippus, Dositheus, Parmeniscus, Conon, Criton, Democritus, and Eudoxus, all agree that the She-Goat[6] rises in the morning of the fourth before the calends of October, and on the third the Kids.
Thrasyllus, the 1st century CE grammarian and astrologer who organized Plato's works into nine tetralogies, included it in the last along with Minos, Laws, and the thirteen epistles.
Modern scholars who have made a case for Plato's authorship include A. E. Taylor and H. Raeder, who, according to Werner Jaeger, “wanted to credit him with the mathematical knowledge it contains”.
Jaeger leans toward the latter view:After Plato died, Philip of Opus, who was his secretary and his Boswell, edited The Laws from his incomplete draft on wax tablets, and divided it into twelve books.
[15]And again:Plato's pupil Philip, who edited the Laws, is certainly echoing his master's thoughts when he says in the Epinomis that mathematical astronomy, the knowledge of the 'visible gods', is an image of the supreme wisdom manifested in them.