The physician U. G. Bucher was a student of Friedrich Hoffmann, who together with Boerhave in Leiden and Stahl in Halle was one of the three major theoreticians of medicine in the early 18th century. The ›Brief-Wechsel‹ (Correspondence), presumably written by Bucher, is the most important contribution of German freethinking to the criticism of spiritualist rational psychology. The standpoint expressed in the ›Brief-Wechsel‹ is a materialism which originated from medical and philosophical sources (Hobbes among others) on the basis of which a sensualistic epistemology is outlined.