“He who does not expect will not find out the unexpected, for it is trackless and unexplored.”
Fragment 18, as quoted in The Art and Thought of Heraclitus: An Edition of the Fragments (1981) edited by Charles H. Kahn, p. 105
Variants:
He who does not expect the unexpected will not find it out.
The Art and Thought of Heraclitus: An Edition of the Fragments (1981) edited by Charles H. Kahn, p. 129
He who does not expect the unexpected will not find it, since it is trackless and unexplored.
As quoted in Helen by Euripides, edited by William Allan (2008), p. 278
Unless you expect the unexpected, you will not find it, for it is hidden and thickly tangled.
Rendering ἐὰν μή "unless" is more English-friendly without being inaccurate. As for the last clause, the point is that you can neither find it nor navigate your way through it. The alpha-privatives suggest using similar metaphoric adjectives to keep the Greek 'feel.' (S. N. Jenks, 2014)
Numbered fragments
Original
ἐὰν μὴ ἔλπηται ἀνέλπιστον, οὐκ ἐξευρήσει
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Heraclitus 46
pre-Socratic Greek philosopher -535Related quotes

“No matter what, expect the unexpected. And whenever possible BE the unexpected.”
Source: Cruddy

“Truly nothing is to be expected but the unexpected.”
As quoted in Alice James, Her Brothers — Her Journal (1934).

dies slowly…
Muere lentamente quien no viaja, quien no lee,
quien no oye música,
quien no encuentra gracia en sí mismo.
Muere lentamente
quien destruye su amor propio,
quien no se deja ayudar...
Poem "Muere lentamente" (Dying Slowly), wrongly attributed to Pablo Neruda. See "Fake Pablo Neruda Poem Spreads on Internet" http://www.laht.com/article.asp?ArticleId=325275&CategoryId=14094 by Ana Mendoza, Latin American Herald Tribune (12 January 2009).
Misattributed
Source: Selected Poems

...y lo que me hace levantarme por las mañanas sigue siendo la espera de lo que está por llegar y no se anuncia, es la espera de lo inesperado, y no ceso de fantasear con lo que ha de venir.
Source: Todas las Almas [All Souls] (1989), p. 137

“A thing long expected takes the form of the unexpected when at last it comes.”

“He who completes a quest does not merely find something. He becomes something.”
Source: The Magician King