Wherever the opportunity is given for definite beings, then they are always there. If a person sends out wrong, evil feelings, these live around him and attract beings which are there waiting, just as some physical being waits for food. I once compared this with the fact that there are no flies in a clean room; if there are all sorts of food-remains in the room, then there are flies. So it is with the super-sensible beings: one need only provide them with the means of nourishment.
Source: Rudolf Steiner – GA 102 – The Influence of Spiritual Beings on Man: Lecture IX – Berlin, 1st June 1908
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