理由
lǐyóu · HSK 3
reason
理由 = 理 (reason/logic) + 由 (from/cause) → the cause behind reasoning
理 carries the sense of logic or principle; 由 means from or owing to. Together they name the from-which of an argument — the grounds or excuse you give. Common in 没有理由 (no reason) and 找理由 (find an excuse).
理 — jade radical 玉/王 + village 里 (phonetic)
理 is a semantic-phonetic compound: the jade radical 王 (玉) on the left signals meaning, while 里 on the right gives the sound. It originally meant the natural veins or grain in a piece of jade, then extended to 'pattern,' 'order,' 'principle,' and finally the modern senses 'reason' and 'to manage.'
由 — pictograph of a gourd or vessel with a stem
由 originated as a pictograph showing a gourd, fruit, or vessel with a sprout or stem emerging from the top. Over time the meaning shifted abstractly to 'come forth from' or 'origin', giving modern senses like 'reason', 'cause', and 'from'. It is unrelated to 田 despite the visual similarity.