他山の石以て玉を攻むべし
(Tazan no ishi motte tama wo osamu beshi;
“One must polish one’s jewels with stones from other mountains.”)

Definition:

A command to take someone else’s mistakes as an opportunity to reflect on and improve your own ways. “One must learn from the mistakes of others.” Just as polishing a gem requires other rocks to act as whetstones, so does the refinement of one’s own skills and character require reference to the foibles, sins, and foolishness of others.

Breakdown:

We begin, slightly into the sentence, with the noun 石 (ishi), “stone.” The particle の (no) associates it with the noun 他山 (tazan), “(an)other mountain.” The direct-object marker is absent here, but assumed, and the action performed upon the stone has taken the form 以て (motte; note the geminate T), “by means of.” (More on that below.) The verb performed by means of the other-mountain stone is 攻む (osamu), in this case “to polish” or “to manufacture.” This appears in conclusive form and is followed by adjective-as-helper particle べし (beshi), “should,” “must,” in sentence-final form.

Keen observers may notice that 以て looks and acts and awful lot like a verb. This is because it is! The verb is 以つ (motsu), in conjunctive form and followed by the particle つ (tsu), also in conjunctive form. The resulting structure is 以ちて (mochite) and as time passed, this was slurred and reduced to the motte that we know today.

Notes:

Because the nuance is specifically of learning from someone else’s flaws and failures, it’s considered an error to use this phrase to refer to learning from a boss or social superior, or even a teacher. Interestingly, it’s also considered incorrect to use it for situations that one doesn’t have a personal connection to – somebody you know, or a situation that impacts your life, can produce 他山の石; something that you merely read about in the news apparently cannot.

Compare and contrast with 反面教師.

This phrase may be shortened to 他山の石, or even rendered into a sort of four-character compound as 他山之石; there is no change in pronunciation or meaning.

As the grammar and word choice suggest, this one’s got some pretty antique origins – specifically, the “Lesser Court Hymns” (小雅, in Japanese shouga) section of the Classic of Poetry (詩経, in Japanese Shikyou), a relatively familiar friend.

Although it doesn’t seem to be the usage in this saying, 他山 can also mean “another temple,” presumably by extension and as a result of the tendency of Buddhist temples to distance themselves from civilian life, and each other, by being sited partway up unclaimed mountain slopes.

Example sentence:

「人のことを馬鹿にしちゃいけないよ。他山の石以て玉を攻むべく、他の人の誤ちを客観的に見て勉強しよう」

(“Hito no koto wo baka ni shicha ikenai yo. Tazan no ishi motte tama wo osamu beku, hito no ayamachi wo kyakkanteki ni mite benkyou shiyou.”)

[“You shouldn’t just make fun of people! You should learn through objective observation of their mistakes, of a mind that the faults of others are good teachers.”]