Several parts combine into one character.

原 shows a spring (泉) welling up beneath a cliff (厂) — its original sense is "the source of a spring." From there grew "origin, source, the original," and the broad plain where the spring flows out split off the sense "field, plain." To make the "source" meaning clearer, 源 (source) was later formed by adding water 水.

Mandarin: yuán, 2nd tone. 原来 (yuánlái, as it turns out / originally), 原因 (yuányīn, cause), 原谅 (yuánliàng, to forgive), 草原 (cǎoyuán, grassland), 高原 (gāoyuán, plateau).

Japanese: on-reading ゲン (gen), kun はら (hara). 原 (hara, plain), 原因 (gen'in, cause), 高原 (kōgen, plateau), 野原 (nohara, field), 秋葉原 (Akihabara). はら is common in Japanese place names and surnames — 原田 (Harada), 藤原 (Fujiwara).

Korean hanja-sound "won": 原因 (wonin, cause), 原理 (wolli, principle), 原來 (wollae, originally), 草原 (chowon, grassland), 平原 (pyeongwon, plain). Both senses — "source" and "plain" — live on.

Memory aid: a spring (泉) welling beneath a cliff (厂) — the source where water begins, and the plain that water waters.