The Vermin only teaze and pinch
Their Foes superior by an Inch.
So, Nat’ralists observe, a Flea
Hath smaller Fleas that on him prey,
And these have smaller yet to bite ’em,
And so proceed ad infinitum.

— Jonathan Swift

Great fleas have little fleas upon their backs to bite ’em,
And little fleas have lesser fleas, and so ad infinitum.
And the great fleas themselves, in turn, have greater fleas to go on;
While these again have greater still, and greater still, and so on.

— Augustus De Morgan