§ 15. Turning now to the doxographic tradition, we may state the problem as follows:—In the _Placita philosophorum_ attributed to Plutarch, in the _Eclogae physicae_ of Stobaeos, in fragments from Arius Didymos, in Hippolytos, and in other writers, we find copious statements as to the _opinions_ of the early philosophers. These opinions shed light on many points not mentioned in the fragments of their writings now remaining, and so they have great importance for the student of their systems. At the same time they are often confused and unreliable. The problem is to determine the relation of these writers to each other, as well as to the source of the whole series, in order that we may estimate their relative value. This work has been most successfully accomplished in the Prolegomena, to Diels’ _Doxographi Graeci_, a work that is absolutely indispensable to the student of this subject. There is no occasion to reopen here a question that Diels has so successfully solved, but I propose to state briefly a few of the conclusions which the reader will find substantiated in the work of Diels. The most obvious fact to one who takes up the study of the doxographic writers is that the _Placita_ attributed to Plutarch, and the _Eclogae physicae_, which was originally a part of the _Florilegium_ of Stobaeos, are intimately related; and when the two are printed side by side, as the reader finds them in the text of Diels, the likeness of the two is most striking. At the same time the two books are not identical, and each gives much material that the other omits. Stobaeos cannot have copied from the work attributed to Plutarch, for even in passages that occur in the _Placita_ Stobaeos not infrequently gives the fuller form; nor can the writer of the _Placita_ have copied from Stobaeos, for his work can be traced back nearly three centuries before the time of Stobaeos. It was used by Athenagoras in his defence of the Christians 177 A.D. (_Dox._ p. 4); it was mentioned by Theodoret (_Dox._ p. 47); and important corrections of the text are made by Diels on the authority of Eusebios, Cyril, and the pseudo-Galen, all of whom had used it. Theodoret (_Therap._ IV. 31, _Dox._ 47) mentions the epitome by Plutarch, but only after he has mentioned the _Placita_ of Aetios, Ἀετίου τὴν περὶ ἀρεσκόντων συναγωγήν, and it is this work of Aetios which Diels vindicates as the source both of Plutarch and of Stobaeos, while Theodoret also quotes from it occasionally. A careful study of these three writers and their methods enables Diels to reconstruct a large part of the work of Aetios; and it is the sections of this work bearing on the earlier philosophers which I have translated (see III. English Index under ‘Aetios’). Of Aetios himself almost nothing is known; the work assigned to him must have been written between the age of Augustus and the age of the Antonines (_Dox._ 100). It was in four books, divided into