If Gèn has appeared in your reading, the book is naming the doubled trigram of mountain — stillness above and below, with no movement to disturb either. The hexagram is one of the four made of a single trigram doubled; like its three siblings (Qián, Kūn, Lí, Kǎn) it names a fundamental state rather than a situation. Stillness here is not the absence of motion. It is the active practice of resting where one is.

Classical commentary reads this hexagram as the most demanding of the meditative configurations. The strange phrase in the judgment — 艮其背, 不獲其身 — keeps still in the back, the self is not perceived — names a state in which the usual sense of being a separate observer has dropped away. There is stillness, and there is no one specifically being still. The hexagram appears when the question is whether such a stillness can be entered, not as performance but as actuality.

What the book counsels is the discipline of not going beyond the actual present situation. 思不出其位 — thought does not exceed its position. This is harder than it sounds. The mind extends easily into the past and the future, into other people's situations, into hypothetical versions of one's own. The hexagram is asking the reader to bring the thinking back to the precise location of the body in the precise moment, and to keep it there.

Gèn's failure mode is the performance of stillness that is in fact a tight controlling of motion. The book is alert to it. The hexagram appears when the reader is being asked for genuine quiet, not the appearance of it. The discipline is the patient one: notice the mind extending, return it without strain, and rest in what is actually here. Done well, the without-blame clause of the judgment follows of itself.