The highest, rational part of the soul, which the Stoics identify with reason (λόγος) itself. It is the seat that forms judgments (ὑπολήψεις), gives or withholds assent (συγκατάθεσις) to impressions (φαντασίαι), and originates impulse (ὁρμή). Human freedom is localised precisely in the ruling part: the external can touch the body, but not the ruling part — unless the ruling part itself has consented.

SVF II 836; DL VII 110; Marc. Aur. Med. 2.2; 05-03; 05-11; 05-26; 06-08; 06-36; 07-05; 07-16; 07-22; 07-28; 07-33; 07-55; 07-62; 07-64; 07-75; 08-03 (the ruling parts of the philosophers "were one and the same" — identical with themselves); 08-43 (my joy is ὑγιὲς τὸ ἡγεμονικόν, the health of the ruling part); 08-48 (the ἡγεμονικόν withdrawn into itself is an unassailable ἀκρόπολις); 08-56 (each ἡγεμονικόν holds its own sovereignty — τὴν ἰδίαν κυρίαν; another's evil does not enter it); 08-61 (to enter another's ἡγεμονικόν and to open one's own — the mutual transparency of ruling parts); 09-07 (ἐφ’ ἑαυτῷ ἔχειν τὸ ἡγεμονικόν — to keep the ruling part in one's own power: the upshot of the four commands); 09-15 (τὸ ἡγεμονικόν pronounces upon things that are themselves mute: the valuation is born in the ruling part, not in the thing); 09-18 (δίελθε ἔσω εἰς τὰ ἡγεμονικὰ αὐτῶν — look inside another's ruling part: the judges you fear judge crookedly themselves); 09-22 (three ἡγεμονικά — your own, the Whole's, your neighbour's: make your own just, remember the whole, acknowledge the kinship); 09-26 (troubles come of not being content with a ἡγεμονικόν that does its own natural work); 09-34 (τίνα τὰ ἡγεμονικὰ τούτων — lay bare the ruling part of the reviler: his verdict carries no weight); 09-39 (the shout to the ἡγεμονικόν itself: τέθνηκας, ἔφθαρσαι, τεθηρίωσαι — are you still alive in your reason?).

In Marcus, the ruling part is a "citadel" (ἀκρόπολις, Med. 8.48) — an inner refuge to which one can withdraw from external impressions. The concept is close to Epictetus' TERMprohairesis but not identical with it: the ἡγεμονικόν is the substrate; prohairesis is its functional aspect.