自由，是沙特無神論存在主義的中軸概念。他提出「存在先於本質」，質疑上帝並不存在，人必須先存在，現身於世上，然後才能自我定義；即使上帝存在，人仍是自我命運的主宰者：人，係由其行為而成就自我，付諸行動，擔負責任。本文以沙特《蒼蠅》劇作為研例，耙梳該劇神話與時代背景並分析劇情與人物；瞭解主角Oreste回到原生城邦，如何面對命定情境之困局，卻能挑戰神旨且意志堅定地完成復仇行動。自由，讓Oreste成為自我的主人，選擇放棄王位，離鄉出走，讓城邦子民重獲自由。沙特劇作中的自由，非求個人己利，實謀人類自由之譜世價值。

Liberté, is the pivotal concept in Jean-Paul Sartre's existentialist antitheism. The idea of l'existence précède l'essence questions the existence of God because human beings have to exist first, to be present in the world in order to define themselves. Even if God does exist, human beings still have to be the master of their destiny. Human beings come to self-fulfillment only through their doings, through taking action (agir) and then taking responsabilité. This essay aims to study Sartre's play, Les Mouches, to analyze its mythical background, plots and characters to show how the protagonist Oreste, when coming back to his native city-state and facing a determined predicament, can challenge God's will and complete revenge with determination. Liberté enables Oreste to decide (choisir) for himself to forsake the crown and leave his homeland, letting the people of the city-state regain liberté to decide (choisir) their own fate. Liberté in Sartre's play is not for personal interests but for the realization of universal liberté for all human beings.