One way to hold this centre: something is tied to the ground and cannot get up. Bound here is the bound of a parcel tied with string. Picture a plane on a runway in a storm, a rocket still on its stand, a bird that has never left the grass. In every case the thing could be expected to rise, and cannot. That held-down feeling is the whole of this centre, and it works just as well on a mind as on a machine.
Wiktionary analyses earthbound as earth + -bound, with earth traced back through Middle English erthe to Old English eorþe. The packet does not say which -bound this is, but both of the word's attested uses show that English has a tied-down -bound and a heading-for -bound.
Wiktionary 把 earthbound 分析成 earth ＋ -bound，earth 一路從古英文 eorþe、中古英文 erthe 傳下來。資料沒有說明是哪一個 -bound，不過這個字被記錄下來的兩種用法本身就顯示：英文有「被綁住」的 -bound，也有「往…去」的 -bound。
Wiktionary entry for earthbound, supplied in packet evidence (source_ref kaikki:3f6f69a190036ae4:1356109800:10561): From earth + -bound.
One branch
被地面綁住，升不上去
一個是身體真的離不開地面，一個是想法離不開地面，畫面完全相同。
adjective · C1
Unable to rise from the surface of the Earth or to leave it. It is used about people, animals and machines that are stuck on the ground, and in ghost stories about a spirit that cannot leave the world.
離不開地面的