Picture a long line of things that move because something ahead of them is moving: the trailer because the truck pulls, the wheels because the engine turns, the campaign because one person kept pushing. Walk to the front of that line. The thing standing there moves everything after it, and nothing moves it. Prime mover names that front position — prime for first, mover for the thing that makes movement.
可以想成一排東西之所以會動，是因為前面那個在動：拖車動，是因為車頭在拉；輪子轉，是因為引擎在轉；一場行動走得下去，是因為有人一直在推。往這排的最前面走。站在那裡的東西帶動了後面的一切，自己卻沒有被誰帶動。prime mover 指的就是那個最前面的位置——prime 是最前面，mover 是讓東西動的那個。
From the word’s history
Wiktionary records the phrase as originally a translation of Latin primum mobile, which is why every use points at whatever stands first in a chain of movement.
Wiktionary 記載這個說法本來是在翻拉丁文 primum mobile。這也是為什麼不論指人、指機器還是指神，它指的都是一條動力鏈最前面的那一個。
Wiktionary, "prime mover"
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