A tree is known by its fruit. 观其果而知其树；观其行而知其人。
据《圣经·新约·马太福音》（Bible, New Testament, Matthew）第12章第33节记载，耶稣·基督（Jesus Christ）说：
“凭着他们（假先知）的果子，你们就能认出他们来。荆棘上岂能摘到葡萄？蒺藜里怎能采到无花果？这样，凡好树都结好果子，坏树则结坏果子。好树不能结坏果子，坏树不能结好果子。凡不结好果子的树，就砍下来丢进火里。”
（You will know them by their fruit. Do men gather grapes from thorn bushes or figs from thistles? Even so, every good tree bears good fruit, but a bad tree bears bad fruit. A good tree cannot bear bad fruit, nor can a bad tree bear good fruit. Every tree that does not bear good fruit is cut down and thrown into fire. ）
在第12章第33节，耶稣又说：“你们或以为树好，果子也好；树坏，果子也坏；因为看果子，就可以知道树。”（for the tree is known by its fruit.）
《圣经·新约·路加福音》（Bible, New Testament, Luke）第6章的第43、44节中，耶稣说：“因为，没有好树结坏果子，也没有坏树结好果子。凡树看果子，就可以认出它来。”（for each tree is known by its own fruit.）在这里，耶稣用树和果子来说明看一个人的好坏要看他行动，也就是“观其行而知其人。”的道理。所以，人们用“A tree is known by its fruit.”来表示“观其行而知其人。”如：
As a man is known by the company he keeps, so a tree by its fruit.
（从所交的朋友就知道他的为人，从所结的果子就知道树的好坏。）
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