During his years in Mainz Leibniz made the acquaintance of the physician Johann Hiskias Cardilucius. Later in his life he claimed that Cardilucius was identical with the alchemist Johann Fortitudo Hartprecht, who had published under the pseudonym “Filius Sendivogii”. By comparing what is known about Cardilucius and Hartprecht it will be shown that Leibniz’s claim is probably correct.