The healthy mind is a perfect sphere (σφαῖρος κυκλοτερής): even, closed upon itself, "at rest" (μονίῃ). In ancient thought the sphere is the ideal form: for Parmenides Being is "like the mass of a well-rounded sphere" (εὐκύκλου σφαίρης, fr. 8); for Empedocles the Sphairos is the cosmos in its blessed unity under Love; for Plato the cosmos is spherical as the "most perfect and self-similar" figure, needing nothing from outside (Timaeus 33b). The sphere has neither beginning nor end, is everywhere equal to itself, bulges out nowhere and is dented nowhere.

Applied to the mind, the image carries a double sense. Invulnerability: nothing "touches" a true sphere (οὐχ ὁτιοῦν ἅπτεται) — not fire, iron, tyrant, or slander; the external does not reach the ruling part so long as it holds its shape. Equipoise: the sphere neither "reaches out" (ἐπεκτείνεται) in craving nor "contracts" (συντρέχει) in fear — either motion would push the sphere out of true; the mind-sphere keeps a perfect poise between greed and avoidance. And it is self-luminous: it shines with its own light, by which it sees the truth, needing no illumination from outside.

Med. 08-41 — the Empedocles quotation: nothing "touches" the ruling part — "not fire, iron, tyrant, or slander — when it has become σφαῖρος κυκλοτερὴς μονίῃ" ("a sphere rounded and at rest"); the image grounds the mind's invulnerability.

• Med. 11.12 — the developed twin: the soul takes on "the form of a sphere" (σφαῖρα εὐκύκλου) when it "neither reaches out toward anything outside nor contracts inward, is neither scattered nor settles, but shines with the light (ἐκλάμπει) by which it sees the truth of all things."
• The philosophical background: Parmenides fr. 8 (Being as εὐκύκλου σφαίρης); Empedocles fr. 27–28 DK (the Sphairos); Plato Timaeus 33b (the sphericity and self-sufficiency of the cosmos).

The image serves the DOGMAinvulnerability of the ruling part and the DOGMAidentification of the "I" with reason: what keeps the sphere's shape is beyond the reach of the external. The practical key (from 11.12) is the two deformations to avoid: stretching out toward the external (craving) and denting inward away from it (fear/aversion); the evenness of the sphere is the TERMinner calm between them. Kindred images are the serene mind and the TERMcalm of the sea (the smooth surface as a figure of rest); the contrast is the mind pushed out of true by the passions, or the marionette whose external strings jerk it this way and that. The self-luminosity of the sphere (11.12) is the same thought as in the joy in the health of the ruling part and in independence from another's opinion (08-44). Kindred too is the mind-as-light image (08-57: self-luminosity — the mind "stands and illuminates" what receives it, while whatever does not pass the light on deprives itself of it; but the light goes forth, whereas the sphere holds its shape inward).