Complex sentence: He retired in 1820, and soon after published one of the best of his works, "Fermenta Cognitionis," 6 parts, 1822--1825, in which he combats modern philosophy and recommends the study of Boehme.
Split into simpler sentences: He retired in 1820, and thereafter published one of the best of his works, "Fermenta Cognitionis" in 6 parts from 1822 to 1825. In it, he combats modern philosophy and recommends the study of Boehme.