For certain reasons, there are opposing, inhibiting forces interwoven in the world plan that want to prevent development. Because we try to develop ourselves through meditation for the advancement of humanity, it is exactly during meditative work that these opposing powers try to interfere in a harmful way: with beginners by pushing to the fore memories of everyday happenings; with advanced seers, by bringing animals or tempting figures in their field of view. These images look like beings one can encounter on the physical plane – one sees mice or rats – but they all stem from a sub-physical region.
Source (German): Rudolf Steiner – GA 266a – Aus den Inhalten der esoterischen Stunden – Munich, March 8, 1909 (page 458)
Translated by Nesta Carsten-Krüger