Diogenes Laertius' work consists of ten books (φιλόσοφοι βίοι, in Phot. Bibl. Cod. cxxi ; φιλόσοφος ἱστόρια in Steph. Byz., σοφιστῶν βίοι in Eustath) and is called in MSS. by the long title of περὶ βίων, δογμάτων καὶ ἀποφθεγμάτων τῶν ἐν φιλοσοφία εὐδοκιμησάντων. According to some allusions which occur in it, he wrote it for a lady of rank (3.47, x, 29), who occupied herself with philosophy, especially with the study of Plato. According to some this lady was Arria, the philosophical friend of Galen (Theriac. ad Pison. 3), and according to others Julia Domna, the wife of the Emperor Severus. (Menage, l.c. ad Prooem. p. 1 ; Th. Reinesius, Var. Lect. 2.12.) The dedication, however and the prooemium are lost, so that nothing can be said with certainty.

The plan of the work is as follows : He begins with an introduction concerning the origin and the earliest history of philosophy, in which he refutes the opinion of those who did not seek for the first beginnings of philosophy in Greece itself, but among the barbarians. He then divides the philosophy of the Greeks into the Ionic--which commences with Anaximander and ends with Cleitomachus, Chrysippus, and Theophrastus--and the Italian, which was founded by Pythagoras, and ends with Epicurus. He reckons the Socratic school, with its various ramifications, as a part of the Ionic philosophy, of which he treats in the first seven books. The Eleatics, with Heracleitus and the Sceptics, are included in the Italian philosophy, which occupies the eighth and ninth books. Epicurus and his philosophy, lastly, are treated of in the tenth book with particular minuteness, which has led some writers to the belief that Diogenes himself was an Epicurean.

Considering the loss of all the numerous and comprehensive works of the ancients, in which the history of philosophers and of philosophy was treated of either as a whole or in separate portions, and a greatnumber of which Diogenes himself had before him, the compilation of Diogenes is of incalculable value to us as a source of information concerning the history of Greek philosophy. About forty writers on the lives and doctrines of the Greek philosophers are mentioned in his work, and in all two hundred and eleven authors are cited whose works he made use of. His work has for a long time been the foundation of most modern histories of ancient philosophy; and the works of Brucker and Stanley, as far as the early history of philosophy is concerned, are little more than translations, and sometimes amplifications, of Diogenes Laertius. The work of Diogenes contains a rich store of living features, which serve to illustrate the private life of the Greeks, and a considerable number of fragments of works which are lost. Montaigne (Essais, 2.10) therefore justly wished, that we had a dozen Laertiuses, or that his work were more complete and better arranged. One must indeed confess, that he made bad use of the enormous quantity of materials which he had at his command in writing his work, and that he was unequal to the task of writing a history of Greek philosophy. His work is in reality nothing but a compilation of the most heterogeneous, and often directly contradictory, accounts, put together without plan, criticism, or connexion. Even some early scholars, such as H. Stephens, considered these biographies of the philosophers to be anything but worthy of the philosophers. His object evidently was to furnish a book which was to amuse its readers by piquant anecdotes, for he had no conception of the value and dignity of philosophy, or of the greatness of the men whose lives he described. The traces of carelessness and mistakes are very numerous ; much in the work is confused, and there is much also that is quite absurd; and as far as philosophy itself is concerned, Diogenes very frequently did not know what he was talking about, when he abridged the theories of the philosophers.

The love of scandal and anecdotes, which had arisen from petty views of men and things, at a time when all political freedom was gone, and among a people which had become demoralized, had crept into literature also, and such compilations as those of Phlegon, Ptolemaeus Chennus, Athenaeus, Aelian, and Diogenes Laertius display this taste of a decaying literature. All the defects of such a period, however, are so glaring in the work of Diogenes, that in order to rescue the common sense of the writer, critics have had recourse to the hypothesis, that the present work is a mutilated abridgment of the original production of Diogenes. (J. G. Schneider in F. A. Wolf's Lit. Anal. iii. p. 227.) Gualterus Burlaeus, who lived at the close of the 13th century, wrote a work , in which he principally used Diogenes. Now Burlaeus makes many statements, and quotes sayings of the philosophers, which seem to be derived from no other source than Diogenes, and yet are not to be found in our present text. Burlaeus, moreover, gives us several valuable various readings, a better order and plan, and several accounts which in his work are minute and complete, but which are abridged in Diogenes in a manner which renders them unintelligible. From these circumstances Schneider infers, that Burlaeus had a more complete copy of Diogenes. But the hope of discovering a more complete MS. has not been realized as yet.

The epigrams, many of which are interspersed in his biographies, and with reference to which Tzetzes (Chil. 3.61) calls him an epigrammatic poet, were collected in a separate work, and divided into several books. (D. L. 1.39, 63, where the first book is quoted.) It bore the title , but, unfortunately, these poetical attempts, so far as they are extant, shew the same deficiencies as the history of philosophy, and the vanity with which he quotes them, does not give us a favourable notion of his taste. (G. H. Klippel, de Diogenis Laertii Vita, Scriptis atque Auctoritate, Göttingen, 1831, 4to.)