Published in two volumes in 1843, Either/Or (original Danish title: Enten - Eller) is an influential book written by the Danish philosopher Søren Kierkegaard, exploring the aesthetic and ethical "phases" or "stages" of existence.

Either/Or portrays two life views, one consciously hedonistic, the other based on ethical duty and responsibility. Each life view is written and represented by a fictional pseudonymous author, the prose of the work depending on the life view being discussed. For example, the aesthetic life view is written in short essay form, with poetic imagery and allusions, discussing aesthetic topics such as music, seduction, drama, and beauty. The ethical life view is written as two long letters, with a more argumentative and restrained prose, discussing moral responsibility, critical reflection, and marriage. The views of the book are not neatly summarized, but are expressed as lived experiences embodied by the pseudonymous authors. The book's central concern is the primal question asked by Aristotle, "How should we live?" His motto comes from Plutarch, "The deceived is wiser than one not deceived." (from Wikipedia)