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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 from life-that is the heart of existentialism.

Walter Kaufmann (2016). “Existentialism From Dostoevsky To Sartre”, p.10, Pickle Partners Publishing
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,