So one sees how the physical is a result of what preceded it spiritually and how in particular circumstances people have it in their power, through knowledge of certain relationships, to connect the physical with its spiritual origin. For example, if one knows how a particular illness is connected with particular feelings and emotions, he knows that by calling up these feelings he can also call up the illness. The black magician can make use of this knowledge to destroy the people. The deep occult truths can therefore not be taught to everyone without due consideration, for it would immediately bring about a sharp demarcation between good and evil. This is the danger inherent in the spreading of occult teachings, for no-one can be taught how to make people well, without at the same time learning how to make people ill.
Source: Rudolf Steiner – GA 93a – THE FOUNDATIONS OF ESOTERICISM – Lecture XXIX – Berlin, 3rd November 1905
Drawing by Marcin Rutkowski