A paradox of self-reference, where, supposing that every man either shaves himself or is shaved by the barber, and the barber shaves all and only the men who do not shave themselves, then it is impossible to determine who shaves the barber.

plural of liar paradox

Grelling-Nelson paradox

(philosophy, logic) A paradox involving statements such as "this sentence is false", or "the following statement is true: the previous statement is false", which cannot be meaningfully regarded as either true or false.

A thought experiment involving a game between two players, one of whom purports to be able to predict the future. The first player has to try to maximize winnings by choosing one of two boxes of money; however, the amount won also depends on whether a prediction made about the choice by the other player proves to be true.

Alternative form of twin paradox

(biology) The apparent paradox that additive genetic variation is maintained despite persistent female choice for particular male traits.

(linguistics) The apparent paradox that children learning English encounter many sentences amenable to dative shift (e.g. "give the book to me" → "give me the book") but apparently have no way to learn that this is not possible with certain verbs (e.g. "*donate me the book" is unacceptable), and yet rarely make this kind of error.

The phenomenon of an arrow travelling in the direction where it points when the bow is fully drawn, when it seems that the arrow would have to pass through its original position. The explanation relates to the bending of the arrow when released.

(philosophy) A philosophical problem whereby a man intends to walk a mile from a particular point, but an infinity of gods separately plan to obstruct him. One of them will raise a barrier to stop his further advance if he reaches the half-mile point, a second if he reaches the quarter-mile point, a third if he goes one-eighth of a mile, and so on ad infinitum. The paradox involves the fact that (i) the man cannot begin walking, because a barrier will stop him even at the shortest distance, but (ii) if he does not begin walking, no barrier will rise.