Attestation. Diognetus is an almost unattested figure in the sources. Two primary testimonies:

1. SHA Marcus 4.9: "studuit et picturae sub magistro Diogneto" — "he also studied painting under the teacher Diognetus." This is the sole direct biographical report apart from Marcus himself.
2. Med. 1.6 (01-06) — Marcus ascribes to him a series of virtues bound up with philosophical initiation: scepticism toward charlatanism, openness to παρρησία, οἰκείωσις with philosophy, a list of further teachers (Bacchius, Tandasis, Marcianus), the writing of adolescent dialogues, the Greek asceticism.

The school affiliation of Diognetus is not established. Birley (2000) conjectures the Platonist school — on the basis of an indirect correspondence between the themes of 01-06 and the Platonist topos of the critique of superstition; Hadot is more cautious, holding the school attribution to be insufficiently grounded. Hard in the OWC commentary: "we know nothing further about him." Farquharson in the 1944 commentary: "scarcely identifiable beyond Marcus's own mention."

Chronology. Per SHA Marcus 2.6 Marcus adopted the ascetic life from the age of twelve (i.e. around 133). If this ascesis is a consequence of Diognetus's teaching, then Diognetus must have been teaching Marcus approximately in 132–133, when Marcus was eleven or twelve. This is before the formal Stoic education of Marcus (Junius Rusticus will come later, in the adolescent years).

The "painting teacher" paradox. SHA ascribes only painting to Diognetus; Marcus ascribes the philosophical conversion to him. This is not a contradiction but a reflection of the fact that the formal status-role of Diognetus in the house of Domitia Lucilla was as teacher of painting (a standard position in an aristocratic household of the second century), while informally he was apparently intellectually close to the philosophical circle and played the part of mentor on that plane. A parallel: Seneca the Elder mentions teachers of rhetoric who imparted to their pupils far more philosophy than of their own formal discipline.

• 01-06 — the sole mention of Diognetus in Marcus.

• SHA Marcus 4.9 — the sole direct biographical testimony.
• A. Birley, Marcus Aurelius: A Biography, rev. ed., Routledge, 2000, ch. 3 ("Education") — the reconstruction of Diognetus's role.
• A. S. L. Farquharson, The Meditations of the Emperor Marcus Antoninus, Oxford 1944, vol. II, ad 1.6 — the standard commentary, recording the scarcity of attestation.
• R. Hard, Marcus Aurelius: Meditations, OWC, Oxford 2011, notes ad 1.6 — the modern summary.
• P. Hadot, The Inner Citadel, tr. M. Chase, Harvard 1998, ch. 2 — the general reconstruction of Marcus's educational biography.