Chicken or the egg

The chicken or the egg is an example of temporal paradox, more specifically an examination of causality, and has also become an expression used in discussing the nature of time.

The question "What came first, the chicken or the egg?" is a paradox, as chickens are hatched from eggs, which are laid by chickens. So the existence of one ensures the existence of the other, but which started the process is a mystery.

The Free Lunch theory answered this question by sugggesting that both the chicken and the egg came into being at the same time. (PROSE: Falls the Shadow)

The Seventh Doctor entertained the Gods of Ragnarok with a magic trick where he drew eggs from his mouth, after first asking "How did life begin? Was it with the chicken, or..." The Gods seemed unfamiliar with this concept. (TV: The Greatest Show in the Galaxy)