A ray as tension, not as leakage. The passage is a model of how Marcus turns Stoic physics into ethics. The key distinction is between χύσις (diffusion, spreading) and ἔκχυσις (effusion, leaking away): the sun is "diffused everywhere" (κέχυται) but has not "flowed out" (ἐκκέχυται). The difference is that the spread of light is τάσις — "tension," the very tonic motion of pneuma by which the Stoics explained the cohesion of the cosmos. Marcus reinforces it with a folk etymology: ἀκτῖνες (rays) "from ἐκτείνεσθαι" (to be extended). The ray does not diminish as it spreads; it stays bound to its source.

The mind should stand, not run out. The vividness of the image — a beam in a shaded room: it travels straight, braces (διερείδεται) against an obstacle, and stands (ἔστη), neither gliding nor falling. Such, Marcus says, should be the "diffusion of the mind" (διάνοια): τάσις, not ἔκχυσις. Against the obstacles (κωλύματα) it meets — no "violent and headlong impact" (βίαιον, ῥαγδαῖον), yet no capitulation either (καταπίπτειν): the mind stands and illuminates what receives it. The discipline of action appears here in the mode of meeting resistance: neither to force through nor to collapse, but to hold the tension.

Whoever shuts out the light deprives himself of the radiance. The closing line shifts the blame to the receiver: αὐτὸ ἑαυτὸ στερήσει τῆς αὐγῆς τὸ μὴ παραπέμπον — "that which does not pass it on will deprive itself of the light." The mind's light is available to all; only the one who has closed himself off stays in the dark. This is the image of MOTIFthe mind as light (the ray as τάσις), to be kept distinct from its neighbours: the mind as fire (4.1 — the flame assimilates whatever falls into it) and the mind as sphere (the self-illumined orb, 8.41). Cf. 12.30 — the sun's light is one, though cut apart by walls: there the same image serves the unity of the rational instead.