The human body is a river (ποταμός): not a static object, but a continuous flow of matter through which the elements stream without ceasing, never remaining the same mass. Every moment it is a different body — the cells have been replaced, the blood renewed, the breath gone. The thing we call "one's own body" is no more numerically identical with itself an hour ago than the river is identical with the current in which one bathed yesterday.

The image is Heraclitean in origin. The famous fragments of Heraclitus:

• ποταμοῖσι τοῖσιν αὐτοῖσιν ἐμβαίνουσιν ἕτερα καὶ ἕτερα ὕδατα ἐπιρρεῖ — "as those who step into the same rivers, other and still other waters flow upon them" (Heracl. fr. 12 DK = B 12, via Cleanthes in Arius Didymus)
• δὶς ἐς τὸν αὐτὸν ποταμὸν οὐκ ἂν ἐμβαίης — "into the same river one cannot step twice" (attributed, fr. 49a)
• πάντα ῥεῖ — "all flows" (not Heraclitus' own formula, but Plato's summary of his teaching, Crat. 402a)

The Stoics absorbed this picture and applied it to the material constitution of the cosmos: every individual thing is a knot of temporary stability in the common stream of the elements, never possessing firm identity at the level of matter. Identity is a function of the organising Logos, not of the substrate.

Med. 2.17 — the most condensed formulation for the body: πάντα τὰ τοῦ σώματος ποταμός ("all that belongs to the body is a river"). The image returns in Marcus often:

• Med. 4.43 — «ποταμός τις τῶν γινομένων ... καὶ ῥεῦμα βίαιον» ("a kind of river of what comes to pass… a violent stream");
• Med. 5.23 — «θεάσασθαι ποταμὸν ὂντα τὰ τοῦ κόσμου» ("to see that all that is in the cosmos is a river");
• Med. 6.15 — "life flows on incessantly, like a stream";
• Med. 7.19 — the whole river is "so small" compared with the whole;
• Med. 12.21 (the closing motif of the entire Meditations).

The Heraclitean background: Heracl. fr. 12, 49a, 91 DK; the reception runs through Plato Cratylus 402a, Theaetetus 152e, 180a–c (the doctrine of universal flow is ascribed to Heraclitus).

In 02-17 the image works in the diagnostic half of the passage — the final catastrophe of human existence:

• the body — a river (ποταμός);
• the psychic — dream and τῦφος (smoke/illusion — a Cynic term, going back to Monimus);
• life — war and a stranger's sojourn;
• posthumous fame — oblivion.

These are seven negative characterisations of human existence in a single sentence, to which Marcus opposes "one thing only" as remedy — philosophy.

The paired image is abscess-on-cosmos (02-16): there the soul is an outgrowth; here the body is a stream. Both show the instability of human "substance," but from opposite angles: 02-16 diagnoses pathological separation from the whole; 02-17 displays natural flow within the whole. The healthy posture is the acceptance of the flow, as in DOGMAτοῦ κατὰ φύσιν: what flows must flow, and resistance to the flow (the fear of dissolution, the fear of death) is resistance to nature.

Connection with the doctrines: the river-image requires the DOGMAontology of the single cosmos as a continuous process and the normative posture of DOGMAlive-according-to-nature. Epistemologically it rests on the TERMdoctrine of transformations and on the physics of the TERMelements: a real "thing" is a temporary configuration of elements in the common cycle.

Heraclitus is a constant interlocutor in Marcus: besides this image, see Med. 4.46 (an explicit citation of Heraclitus), 6.42 (a dialogue with Heraclitus on sleep), 8.3 (Heraclitus together with Socrates, Pythagoras, Archimedes, and Alexander — all dead), and 03-03 (Heraclitus' death by dropsy).