“A frog in a well cannot conceive of the ocean.”

Letter to David Hilbert (2 October 1897)
Context: The totality of all alephs cannot be conceived as a determinate, well-defined, and also a finished set. This is the punctum saliens, and I venture to say that this completely certain theorem, provable rigorously from the definition of the totality of all alephs, is the most important and noblest theorem of set theory. One must only understand the expression "finished" correctly. I say of a set that it can be thought of as finished (and call such a set, if it contains infinitely many elements, "transfinite" or "suprafinite") if it is possible without contradiction (as can be done with finite sets) to think of all its elements as existing together, and to think of the set itself as a compounded thing for itself; or (in other words) if it is possible to imagine the set as actually existing with the totality of its elements.

Restriction
One Minute Wisdom (1989)

By The River Piedra I Sat Down and Wept (1994)
Variant: There's nothing deeper than love. In fairy tales, the princesses kiss the frogs, and the frogs become princes. In real life, the princesses kiss princes, and the princes turn into frogs.
Source: By the River Piedra I Sat Down and Wept

"Quotations by 60 Greatest Indians" at Dhirubhai Ambani Institute of Information and Communication Technology http://resourcecentre.daiict.ac.in/eresources/iresources/quotations.html

“The Teachings of Don B.: A Yankee Way of Knowledge”, pp. 7–8.
The Teachings of Don. B: Satires, Parodies, Fables, Illustrated Stories, and Plays of Donald Barthelme (1992)

Source: Eat That Frog!: 21 Great Ways to Stop Procrastinating and Get More Done in Less Time

Variant translation: Boys throw stones at frogs for fun, but the frogs don't die for "fun", but in sober earnest.
As quoted by Plutarch, Moralia, xii. 66

"The Celebrated Jumping Frog of Calaveras County" (1865)