Original: Considered by many as a founder or precursor of 20th-century existentialism, his Notes from Underground (1864), written in the embittered voice of the anonymous "underground man", was called by Walter Kaufmann the "best overture for existentialism ever written.
Simplified: He is sometimes considered to be a founder of existentialism, most frequently for Notes from Underground, which has been described by Walter Kaufmann as the best overture for existentialism ever written.