What does it mean, 'inner' and 'outer trigram' (popup)
Thunder below Earth: Thunder is the voice of Heaven, the knowledge of how things should be or should move. The Earth is the wide space where everything happens. It is easy to forget the base of what is happening, because with everything one does one looks forward. It is very important to look back from time to time, or to look inward. Is this what I am, is it what I really want?
Ideogram of the hexagram name: at the bottom a foot, going backwards, above it a picture of a city. Returning to one's place or one's origin. Restore what was before, or start again at the beginning.
The 'Great Image' says: Thunder in the center of the Earth: return. The first kings closed the frontiers at sun solstice, peddlers did not travel, the empress did not leave the region.
Hex.24 is the contrast of 23, which is about changing things with an eye upon the future, removing what is not right anymore or what might grow into something wrong.
Hex.24 is returning to the core, recovering the essential.