His researches deal with the Bible, Talmud and Midrash, Jewish religious philosophy, and Jewish history. In his most important work, Die Koenigsgleichnisse des Midrasch (1903), he dealt with the "parables of kings in the Midrash" in the light of the historical and factual reality of the Roman Empire. His other important works are: Religioese Disputationen im Mittelalter (1894); Die Geschichte des Judenthums (1900); Die Geistesreligion und das juedische Religionsgesetz (1912); Dokumente zur Geschichte der Juden in Karlsbad (1791–1869) (1913); Das magische Judentum (1923); and the two-volume Die sittliche Welt des Judentums, 2 vols. (1924–28).