SYNKATATHESIS is ‘assent’. In Stoicism, it refers to the act of approving impressions, conceptions, and judgments which therefore prompts action to take place.
This is the third level of self-coherence, concerning the will/judgement and what we choose to reject. In fact, <synkatathesis> is part of a sequence whose task is for us to face reality. Firstly, there is <aisthesis> which results from the activation of our senses. For instance, if a stone hits our face a set of sensations is immediately operating and we receive several stimuli.
Secondly, there is <phantasia> which is the representation of the stimuli when passing from senses to soul. As an example, while <aisthesis> gives us the sensations of pain (or penetration, piercing, smashing, hurting and so on), <phantasia> gives us the idea of an object hitting our face, maybe not too big, possibly a stone. <Phantasiai> are produced not only by external objects but also by internal conditions, like virtue or vice. Nonetheless, these two events (sense impressions and soul representations) are mainly of a passive kind. Our free choice is not involved.
Thirdly, there is <synkatathesis>. This is the <logos> inside ourselves (as a mirror of the universal <logos>) deliberating to give assent or not (or even suspending the judgement) to representations. In this third case a choice is in play, although the same choice depends on the nature of the representation.
<Synkatathesis> is ‘assent’. But ‘assent’ itself is not entirely our choice. In other words, our conscience cannot dissent in front of the “smashing” evidence of a stone hitting our face (maybe thinking it is just a water downpour). When something like that happens, it means we are mad or anyway not in control of ourselves (for instance, because of drugs or alcohol).