The argument from craft. Marcus takes an image from the workshop: even the "vulgar" craftsman (βάναυσος τεχνίτης — one who works for hire) yields up to a point to the taste of the unskilled (ἰδιῶται), yet does not abandon the reason of his craft (TERMλόγος τῆς τέχνης — the rule, the principle of the trade): to betray it is unbearable to him. The governing discipline is assent (fidelity to right reason); the sustaining one is action (the analogy with craft and duty).

A fortiori. Hence the "how much more" inference: if the architect and the physician so respect (αἰδέσονται) the TERMλόγος of their particular art, is it not shameful for a human to respect his own reason less? A craft is only a particular λόγος; human reason is the universal one, the very thing for which a human is human.

Reason common with the gods. The climax is in the subordinate clause: this reason is "DOGMAcommon to him and the gods" (κοινὸς πρὸς τοὺς θεούς). Human λόγος is not a private faculty but a portion of the one cosmic Reason; therefore to hold to it is not pedantry but fidelity to the divine within. Hence the tag of DOGMAthe unity of the cosmos: one λόγος pervades gods and human beings, making them fellow-citizens (cf. cosmopolis).

Parallels. Med. 4.4 (common reason → common law → we are fellow-citizens, the cosmopolis); Med. 7.53; Epictetus on fidelity to "the art of living." Cf. 06-30 (to keep in oneself what "philosophy wished to make" one).