After Cohen's death, Cassirer developed a theory of symbolism and used it to expand phenomenology of knowledge into a more general philosophy of culture. Cassirer was one of the leading 20th-century advocates of philosophical idealism. His most famous work is the ''Philosophy of Symbolic Forms'' (1923–1929).
Though his work received a mixed reception shortly after his death, more recent scholarship has remarked upon Cassirer's role as a strident defender of the moral idealism of the Enlightenment era and the cause of liberal democracy at a time when the rise of fascism had made such advocacy unfashionable. Within the international Jewish community, Cassier's work has additionally been seen as part of a long tradition of thought on ethical philosophy.