MESSENGER: Nothing is more useful to mankind than a prudent distrust.

[ἌΓΓΕΛΟΣ:σώφρονος δ᾽ ἀπιστίας
οὐκ ἔστιν οὐδὲν χρησιμώτερον βροτοῖς.]

Euripides (485?-406? BC) Greek tragic dramatist

Helen [Ἑλένη], l. 1617ff (412 BC) [tr. Coleridge (1891)]
    (Source)

Informing Theoklymenos that Helen and Menelaos have escaped to Greece.

(Source (Greek)). Other translations:

Nought to man's welfare more
Avails, than disbelief by prudence rul'd.
[tr. Potter (1783), l. 1750ff]

There's nought more beneficial to mankind
Than wise distrust.
[tr. Wodhull (1809)]

But there is naught more useful to mortals than a wise distrust.
[tr. Buckley (1850)]

Nought is of more avail
For mortals' need than wise mistrustfulness.
[tr. Way (Loeb) (1912)]

My lord, I fear
There are advantages in honest doubt.
That lesson of my tale at least is clear.
[tr. Sheppard (1925)]

I say there's nothing of more use
to mortals than a wise suspension of belief.
[tr. Warner (1951)]

Well, there is one thing every man has to learn: it is, not to be too trustful.
[tr. Vellacott (1954)]

Man's most valuable trait
is a judicious sense of what not to believe.
[tr. Lattimore (1956)]

Knowing when not to believe someone ...
now that's the kind of wisdom we could use!
[tr. Meagher (1986)]

To sum up, there’s nothing more useful in life than showing a healthy scepticism.
[tr. Davie (2002)]

Ah! there is naught more serviceable to mankind than a prudent distrust.
[tr. Athenian Society (2006)]

Seems you were a bit naive, sir:
There's nothing more useful in life
Than a good suspicious nature.
[tr. A. Wilson (2007)]

Nothing is more useful than a prudent doubt.
[tr. Theodoridis (2011)]

Nothing is more useful to man than thoughtful skepticism.
[Ambrose et al. (2018)]

Nothing is more useful to mankind than a balanced [sōphrōn] distrust.
[tr. Coleridge / Helen Heroization Team]