Following his conviction that the truth, in principle, could only be one truth, Leibniz wanted to prove that the “inner Light” of “true mysticism” was compatible with philosophers’ insights into reason. He placed his own doctrine of the individual substances (“Monadology”) in the tradition of philosophia perennis, and in his opinion “true” mysticism belonged to this same tradition. Through representatives of the so-called Christian Kabbala, which made Jewish and Christian mysticism part of the same lineage, Leibniz became aware of the controversial writings of Böhme. He discussed the theosopher’s thought with his correspondence partner, the Böhme adherent André Morell, and after a critical investigation, Leibniz placed Böhme in a tradition of “true mysticism” which could be measured against reason. The author posits that the result of this engagement was Leibniz’s treatise “True Mystical Theology”, a translation of his doctrine of monads in mystical terminology.