Synkatathesis literally means "joint deposition" (from σύν- + κατά + τίθημι, "to place down together"); it is rendered "assent" or "affirmation." In Stoic epistemology it is the central act of cognition: the TERMruling part's assent to a TERMimpression, turning the impression into an assertion that "this is such-and-such."

The Stoic cognitive chain:

1. Phantasia (impression) arrives in the soul — passively, not up to me.
2. Synkatathesis — the ruling part gives or withholds assent — actively, in my power.
3. If assent is given, a TERMὑπόληψις (a fixed opinion) is formed.
4. From the ὑπόληψις, if it is an evaluative judgment, an TERMὁρμή (impulse to action) follows — or, if the assent is false, a TERMπάθος (passion).

Assent is the only properly moral action in the Stoic system. I cannot choose which impression will appear to me, which blow of fortune will fall, which pain will enter the body. But I can choose whether to assent to the evaluative content of the impression. This microscopic point of freedom is the foundation of the whole of Stoic ethics.

Assent is of two kinds:

• Assent to a καταληπτικὴ φαντασία (a cognitive impression) — the only valid kind. The sage gives assent only to what bears witness to itself as true.
• Assent to a non-cognitive φαντασία — a "weak opinion" (ἀσθενὴς ὑπόληψις), δόξα; the source of all error and of all passions.

The Stoic ideal is never to give assent to weak impressions. In Epictetus this becomes the practice of "epochē of evaluative judgments": waiting until the impression has certified itself.

SVF I 60–61 (Zeno on συγκατάθεσις); II 53–70, 99–105 (Chrysippus' extensive development); II 870, 974 (the role of συγκατάθεσις in the formation of ὁρμή); DL VII 49–54; Stob. Ecl. II 88, 111 W; Cic. Acad. I 40–42, II 38 (ἐπέχειν — the suspension of assent); LS 39–41, 53. The term συγκατάθεσις is rare in Marcus (he does not cite school terminology strictly), but the concept is hard at work: Med. 5.27; 8.7; 8.49 ("add nothing of yourself to what the φαντασία says" — this is the refusal of unwarranted συγκατάθεσις); 11.3.

Synkatathesis is the technical heart of Stoic ethics. The famous formula «πᾶν TERMὑπόληψις» of 02-15 reduces operationally to «πᾶν συγκατάθεσις»: everything that depends on me is the set of assents my ruling part gives. From Epictetus onward: "people are disturbed not by things, but by their opinions about things" (Ench. 5) — and opinions are formed through συγκατάθεσις.

Connection with Hadot's three disciplines:

• Discipline of assent — literally work on συγκατάθεσις: watching which impressions we affirm and which we reject.
• Discipline of desire — governing one's orientation toward the good by preventing false evaluative συγκατάθεσις.
• Discipline of action — ensuring that the συγκατάθεσις given to an impression of what ought to be done passes critical scrutiny.

See also the exercises παρακολούθησις (attentive monitoring of one's own impressions and assents) and physical definition (the stripped-down view of things that drains their attractiveness → no false assent).

In its radical form, the suspension of assent brings the Stoic and Sceptical positions close: Carneades, taking over the term ἐπέχειν, developed it into a system of universal suspension. The Stoics are not so radical — they allow assent to καταληπτικαὶ φαντασίαι — but in practice they teach a very cautious granting of assent.