Ernst Alfred Cassirer (/kɑːˈsɪərər, kəˈ-/ kah-SEER-ər, kə-; German: [ˈɛʁnst kaˈsiːʁɐ]; July 28, 1874 – April 13, 1945) was a German philosopher and historian of philosophy. Trained within the Neo-Kantian Marburg School, he initially followed his mentor Hermann Cohen in attempting to supply an idealistic philosophy of science.
After Cohen's death in 1918, Cassirer developed a theory of symbolism and used it to expand the "logic and psychology of thought" into a more general "logic of the cultural sciences". Cassirer was one of the leading 20th-century advocates of philosophical idealism. His most famous work is the Philosophy of Symbolic Forms (1923–1929).