Picture a small pale light drifting over wet ground at night. You walk toward it; it is still the same distance away. You walk further; the ground is softer now, and the light has moved again. That one scene holds the whole word. Whether it is gas over a marsh or a goal you have been chasing for years, the shape is the same: something visible in front of you that you can follow forever and never stand next to.
The source entry gives Will (a man's name) + of + the + wisp (a bundle of straw used as a torch) — the light was named as if a man called Will were carrying it.
來源資料把它拆成 Will（男生的名字）＋ of ＋ the ＋ wisp（當火把用的乾草把）。也就是說，古人是把那團光當成「有個叫 Will 的人提著草火把」。這個構詞本身就說明了為什麼它的比喻義這麼自然：它從一開始就是一個會走、會引路的東西，而不是一個死的光點。
Wiktionary extract supplied in packet (source_ref kaikki:3f6f69a190036ae4:734072018:46119)
Two branches
沼澤上飄的那團光
這一支就是那個畫面本身：看得見的一團光，加上英國舊傳說裡「那是妖精的燈籠」的說法。
noun · C2
A pale, moving light that can be seen over wet ground at night, made by natural gases from the marsh. In old British stories it is a lantern carried by a spirit to lead travellers off the path.
鬼火；沼澤上的磷火
追不到的目標、靠不住的人
把「朝那盞光走，它一直退開」整個搬到人生上，就得到這一支；連結是比喻上的，不是另一條字源。
noun · C2
Something you keep going after but can never reach, or someone you can never really depend on. The thing or person seems to be there, and then it moves away again.
追不到的目標；靠不住的人