(Tazan no ishi motte tama wo osamu beshi;
“One must polish one’s jewels with stones from other mountains.”)
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.
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.
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.
(“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.”]