The Flies explores the fundamental difference between man and everything else in the universe. According to Jean-Paul Sartre and his existentialism, man exists in a way entirely different from other forms. Man exists as being-for-itself (a conscious and active form of being). Creatures, objects, plants, nature, and anything else exist as being-in-itself (a passive, unconscious mode of being). Because of this difference, man exists outside of and separate from all of nature. Man chooses how to interpret nature and what value it will hold for him; but man is not a part of nature.
Biological instincts conflict with human freedom in Sartre's The Flies.