Heraclitus of Ephesus was an Ionian philosopher of the turn of the 6th–5th centuries BCE, of a noble priestly family (by tradition descended from Androclus, the city's founder). He is said to have ceded the hereditary office of "king" (βασιλεύς, a priestly magistracy) to his brother. He was nicknamed "the Obscure" (ὁ Σκοτεινός) and "the weeping philosopher" — for his deliberately enigmatic, aphoristic style and his contempt for the crowd. His work Περὶ φύσεως ("On Nature") he is said to have deposited in the temple of Artemis at Ephesus, writing it intentionally darkly so that only the worthy might reach it.

The principal biographical source is Diogenes Laertius IX 1–17. From the same place come the circumstances of his death cited by Marcus (03-03): Heraclitus fell ill with dropsy (ὕδρωψ), smeared himself with cow-dung (βόλβιτον) in the hope that its warmth would draw out the moisture, and died. The ancient irony is plain: the philosopher of fire perishes from an excess of water.

Heraclitus is a pivotal figure for the Stoics; it is from him that the Stoa draws the foundations of its physics. The key doctrines:

• Λόγος — the single rational law-measure that governs all; "all things come to pass according to this logos" (fr. B1). Human beings live "as if asleep," unaware of the common logos. The Stoic cosmic λόγος is the direct inheritance of this teaching.
• Πάντα ῥεῖ — "all things flow": ceaseless change as the nature of what is; "you cannot step into the same river twice" (B12, B49a, B91). See the image the body as a stream, which Marcus uses repeatedly.
• Πῦρ — fire as the first principle and the image of the cosmic process: "this cosmos … was, is, and ever shall be an ever-living fire, kindling in measures and going out in measures" (B30). Hence the Stoic doctrine of the periodic conflagration (ἐκπύρωσις) and of eternal recurrence.
• The unity of opposites — the way up and the way down are one and the same (B60); day and night, life and death, are one.
• Πόλεμος πατὴρ πάντων — "war is the father of all" (B53): the tensile strife of opposites as the condition of order.

For understanding Marcus, Heraclitus matters more than any other Presocratic: the Stoic λόγος, ἐκπύρωσις, the cosmic fire-pneuma, and the doctrine of ceaseless transformation all go back to him.

• 03-03 — death by dropsy in the catalogue of famous deaths: "after so much speculation on the conflagration (ἐκπύρωσις) of the universe, he died filled inside with water." The lesson: the deepest φυσιολογία does not exempt the body from a wretched end.
• Med. 4.46 — the most developed treatment: Marcus quotes Heraclitus directly (the death of earth is to become water, of water to become air; "they are at variance with the logos with which they most constantly associate"; "we ought not to act like children of our parents").
• Med. 6.42 — the image of sleepers, who too are fellow-workers in the cosmic process (the Heraclitean motif of sleeping and waking).
• Med. 6.47 — in the roll-call of the dead.
• The river/stream image — Med. 2.17, 4.43, 5.23, 6.15, 7.19, 9.29 [verify:med]; see the card body-as-river.