Søren Kierkegaard's monumental work, 'Either/Or: A Fragment of Life,' delves into the core of human existence through two opposing philosophies: the aesthetic and the ethical. Presented as papers by pseudonymous authors, the book explores a life of sensory pleasure versus one of moral responsibility. Kierkegaard masterfully contrasts these worldviews, compelling the reader to confront the profound question of how one ought to live, making it a foundational text of existentialism that challenges us to make a definitive, personal choice.