Genre and place in the book. A one-line maxim-rule, in the clipped manner of Book VI (cf. 06-02). It is a concentrated formula of the exercise of EXERCISEphysical definition: let no thing slip past you without being grasped in its real constitution and its real worth. The principal discipline is assent (the exact apprehension of an impression); the sustaining one is desire (correct valuation).
"Look within" (Ἔσω βλέπε). The gaze is directed not at the surface but into the thing (Rogovin, plainly: "do not be content with a surface look"). Marcus teaches one to penetrate the drapery of custom and passion to what the thing actually is. It is worth noting: Long renders "Look within" identically here and at 7.59 — but there the verb and the object differ: Ἔνδον σκάπτε, "dig within [yourself]: there is the fountain of good." In 6.3 one looks into the inside of things (their nature and worth); at 7.59, into oneself (the spring of good). One English phrase, two different exercises.
The peculiar quality (ἰδία ποιότης). The first thing that must not escape is the "peculiar quality" of a thing, what makes it this thing and no other. Behind the phrase stands the Stoic doctrine of individuation: each existing thing is an ἰδίως ποιόν, an "individually-qualified" being, distinguishable from every other by its unique quality (LS 28). To grasp the ἰδία ποιότης is to put the thing in its own class, not confusing it with what it merely seems.
Worth (ἀξία). The second is the true TERMworth of the thing (ἀξία). This is the technical term of Stoic axiology: an indifferent (ἀδιάφορα) has its own axia — positive (προηγμένα) or negative (ἀποπροηγμένα) — but it does not thereby become a good or an evil. Marcus's demand: to value each thing by its actual worth, neither inflating it (as passion does) nor deflating it. Correct axia regulates rational action and desire; hence the discipline of desire trails the discipline of assent here.
Disciplines and parallels. The principal one is assent (to see the thing as it is, not to let its quality escape); the secondary is desire (to value it at its true worth). Parallels: 03-11 (the method of definition: to describe the object, its composition, and its axia within the whole universe); Med. 8.11 ("what is this in itself, in its own constitution? what is its substance, what its cause?"); 6.13 (the bare definitions — Falernian wine, purple, coitus). Cf. the exercise of EXERCISEphysical definition and the term TERMἀξία.