Boris Schatz · Public domain

Founded Chabad, a branch of Hasidic Judaism built around applying mystical ideas to everyday intellectual and emotional life, and became its first Rebbe. His central work, the Tanya, laid out a systematic psychology of the soul's struggle between selfishness and spiritual purpose that Chabad still teaches as its core text. He also wrote the Shulchan Aruch HaRav, a restatement of Jewish law, and compiled his own prayer book.