It is absolutely possible for an individual to exhibit qualities which compel us to treat him as mentally inferior, feeble-minded: nevertheless the same person may utter things — which are full of life and wit to the point of genius. That is quite possible. And why? Because of the extreme suggestibility associated with certain types of mental inferiority; a suggestibility open to all the mysterious influences of the environment and reflecting them as a mirror.
Source: Rudolf Steiner – GA 312 – Spiritual Science and Medicine: Lecture XIII – Dornach, 2nd April 1920