The refusal to belong to any school of thought, the repudiation of the adequacy of any body of beliefs whatever, and especially of systems, and a marked dissatisfaction with traditional philosophy as superficial, academic, and remote — that is the heart of existentialism.

Existentialism from Dostoevsky to Sartre
Walter Kaufmman
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Some Key Existential Philosophers

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From left to right, top to bottom: Søren Kierkegaard, Fyodor Dostoevsky, Friedrich Nietzsche, Jean-Paul Sartre