The Vita of the emperor in the collection known as the Historia Augusta identifies him in its heading as Marcus Antoninus Philosophus, "Marcus Antoninus the Philosopher." Toward the end of the work, the following is reported about him, sententia Platonis semper in ore illius fuit, florere civitates si aut philosophi imperarent aut imperantes philosopharentur (27.7), "Plato's judgment was always on his lips, that states flourished if philosophers ruled or rulers were philosophers." It is this quality of Marcus' character which has made him a unique figure in Roman history, since he was the only emperor whose life was molded by, and devoted to, philosophy (Julian was the second and last). His reign was long and troubled, and in some ways showed the weaknesses of empire which ultimately led to the "Decline and Fall," yet his personal reputation, indeed his sanctity, have never failed of admirers. Contributing to his fame and reputation is a slender volume of Stoic philosophy which served as a kind of diary while he was involved in military campaigns, the Meditations, a book which can be described as an aureus libellus, a little golden book.

The sources for understanding Marcus and his reign are varied but generally disappointing. There is no major historian. The chief literary sources are the biography in the Historia Augusta, as well as those of Hadrian, Antoninus, Verus, and Avidius Cassius. Debate about this collection of imperial biographies has been heated and contentious for more than a century. In all likelihood, it is the work of a single author writing in the last years of the fourth-century. The information offered ranges from the precisely accurate to the wildly imaginative.

Cassius Dio, who wrote in the decade of the 230s, produced a long history of the empire which has survived, for our period, only in an abbreviated version. Fourth century historians, such as Aurelius Victor and Eutropius, occasionally furnish bits of information. Marcus' teacher, Fronto, a distinguished orator and rhetorician, is extremely useful. Papyri, inscriptions, coins, legal writings, and some of the church writers, such as Tertullian, Eusebius, and Orosius, are very important. Archaeology and art history, with their interpretation of monuments, make the history of Marcus' principate literally visible and offer important clues for understanding the context of his actions..