Pneuma is the central concept of Stoic physics: a subtle "fiery breath," a mixture of fire and air, pervading the whole cosmos and binding it into a single whole (συμπάθεια). In the human being the Stoics distinguish several grades of pneuma: φυτική (vegetative — growth), ζωτική (animal — vital force), αἰσθητική (perceptive), and λογιστική (rational), with the highest layer identical with the TERMruling part and the TERMintellect. In the narrow, bodily sense, pneuma is simply breath — ephemeral and constantly exchanged.

SVF I 134–144 (Zeno's doctrine of the pneuma); II 439–462 (Chrysippus on the four grades of pneuma); II 773–800 (pneuma as "the body of the soul"); DL VII 156–157; LS 47, 53. In Marcus: Med. 2.2; 4.41; 6.15; 9.2; 09-36 (πνευμάτιον, the diminutive: the "little breath" too is "another such thing", passing from this into that); 10-07 (τὸ πνευματικόν turns into the airy and is taken back into the λόγος of the Whole; and it was gathered from inhaled air "yesterday and the day before"); 12.30. In Book XII — 12-03 (πνευμάτιον, the second of the three components, "grown together" with the body (σύμφυτον), and yours only so far as it must be cared for). And 12-14 (the πνευμάτιον is surrendered to the surge along with the σαρκίδιον: everything may be carried off except the νοῦς). And 12-30 (πνεύματα are put among the things that are ἀνοικείωτα ἀλλήλοις: what holds them is not kinship but ἕξις and weight). In Books I–IV: 04-03, 04-04 (τὸ πνευματικόν — the life-giving element, common to all).

In 02-02 Marcus uses precisely the diminutive πνευμάτιον ("little breath") and immediately reduces it to bare physics: "a breath that does not stay self-identical, exhaled and inhaled again hour by hour." This is not a denial of the Stoic doctrine of higher pneuma as rational — it is the inverse, ascetic move: so long as attention dwells on breath as a biological process, breath is stripped down to mere matter; the "I" proper is the TERMruling part, the third and only significant component. See unity-of-cosmos (pneuma as cosmic bond) and definitio (the exercise of the "stripped-down view"). See also 11-20 (the πνευμάτιον and the fiery tend upward by nature yet are held in the compound — an a fortiori argument: the elements endure a station not their own, while the rational part will not endure its own).