A monster under every bed
(Hito wo mitara dorobou to omoe;
“When you see someone, think them a thief”)
Don’t blindly trust strangers. A proverb advising the listener to protect themselves by doubting and confirming, rather than naively believing the best and accepting whatever people say. “Assume everyone you meet is a thief.”
To be honest, I don’t like this as a rule for day-to-day, in-person life. Most people are just folks, and don’t mean any particular harm. That said, this should probably be Rule One on the internet, where anonymity plus facelessness plus unlimited range make everything more dangerous and less trustworthy.
We begin with the noun 人 (hito), “person,” although in this case it carries the nuance of “other person,” “outsider,” “stranger,” rather than people in general. This person is marked by the particle を (wo) as the object of the verb 見る (miru), “to see,” in conditional form, acting as “when.”
The clause that describes the result of this condition being met begins with the noun 泥棒 (dorobou), “thief,” “robber,” marked by the particle と (to, sounds like “toe”). This turns the part before the と into something like a quoted bit of text – in this case, the content of the following verb. This verb is 思う (omou), “to think,” in imperative form.
Keep in mind that this saying should not be taken literally: don’t go around assuming that everyone is a criminal. Instead, the point is to be cautious and reserved in your dealings with others, especially people you don’t know well.
Several variants use synonyms such as 盗人 (nusubito) instead of 泥棒. A couple of variants also warn you to think of every fire as a dangerous conflagration (火事 = kaji, or 焼亡 = joumou) – again, the point is to encourage a healthy level of caution. Yet another variant advises you to think that 明日は雨 (ashita wa ame) – “tomorrow it will rain.”
Contrast with 七度尋ねて人を疑え and 渡る世間に鬼は無し.
Exactly one of my sources asserts that this phrase comes from a kabuki play titled 『楷子乗出初晴業』 (Hashigo-nori dezome no harewaza) by popular Edo-era playwright 河竹黙阿弥 (Kawatake Mokuami).
(“Daigaku ichinenme wa shiranai machi de aka no tanin ni kakomareganara no hitori-gurashi datta shi, oya ni iwareta toori ni hito wo mitara dorobou to omotte oita. Ittai, dore dake no sagishi wo sakerareta no ka, dore dake atarashii nakama wo tsukuru kikai wo nogashite shimatta no ka wa, furikaette mite mo sappari wakaranai.”)
[“My first year of college was an isolated life, surrounded by complete strangers in an unfamiliar town, so I took my parents’ advice and treated everyone I met as a potential robber. Looking back now, I don’t have the slightest idea how many con artists I was able to avoid that way, or how many chances I missed to make new friends.”]