Final answer:
Jean-Paul Sartre's existentialism is encapsulated in the key slogan "existence precedes essence", asserting that individuals define their essence through choices and actions in a world without inherent meaning.
Step-by-step explanation:
The primary slogan of existentialism, according to Jean-Paul Sartre, is "existence precedes essence". This core belief posits that an individual first exists without predefined characteristics or purpose, and only later defines their essence through their actions and decisions. This is central to the philosophy of existentialism, which emphasizes individual freedom and responsibility, and challenges the notion that humans have inherent essence or predetermined purpose in life. Sartre's existentialist perspective is characterized by his delineation of two types of beings: being-for-itself (conscious and free) and being-in-itself (non-conscious). As per Sartre, humans are in a constant process of self-creation, and through their existential freedom, they bear the heavy burden of responsibility to define their own lives amid the absence of inherent meaning.