蛇の道は蛇
(Ja no michi wa hebi; “Snakes [know] the snakes’ road”)

Definition:

People who belong to a certain group are the ones who know the most about the ways of that group. Specialists understand other specialists in their field better than anyone else could. “Set a thief to catch a thief.”

Breakdown:

蛇 is “snake,” but here it’s pronounced two different ways. The first is ja, with the nuance of a large snake; the second is hebi, implying a small one. In between these two snakes is 道 (michi, “street,” “road,” “path,” “way,”), connected to the first with the associative particle の (no). は (wa) is the topic-marker particle. The result becomes roughly, “As for snake’s path, snake.” (But see below.)

Notes:

The phrase can be completed by adding ～が知る (ga shiru), a subject marker plus a sentence final form of a verb approximate to the English “to know.” This longer form thus becomes “Snakes know the ways of snakes.”

As mentioned above, ja implies a large snake, while hebi is a smaller one. Supposedly the saying originally referred to the belief that smaller snakes would travel along the trails left by larger ones – meaning that the hebi would be well acquainted with the “ways” of the ja. If you want to know “the path of the ja,” then, you would do well to ask a hebi.

Keep this in mind, because while the common negative interpretation of “snake” fits well with the given English-language rendition of “Set a thief to catch a thief,” ja isn’t just a large snake: it can also be a dragon. (For example, the Nagasaki Okunchi festival features a Chinese-style “dragon-dance” called 蛇踊り – ja odori, “ja dance” – usually “dragon dance.”) It was believed that a snake that lived long enough would grow and develop into dragonhood, spending a thousand years in the sea and a thousand years in the mountains in the process. Think of how long and sinuous Eastern Dragons are. And maybe next time you meet a little green snake, show it a little more respect.

Example sentence:

「『羊たちの沈黙』という映画の前提は一言に言えば、『蛇の道は蛇』らしい」

(“’Hitsuji-tach no chinmoku’ to iu eiga no zentei wa hitokoto ni ieba, ‘ja no michi wa hebi‘ rashii”)

[“Summing up the premise to the Silence of the Lambs movie in a single phrase, I’d say it’s ‘Set a thief to catch a thief.’”]