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; 5.11; 8.48.

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.