It is folly simply to use the expression “mental disease”; the spirit is always healthy, and cannot fall sick in the true sense of the term. To talk of mental diseases is sheer nonsense. What happens is that the spirit’s power of expression is disturbed by the bodily organism, as distinct from a disease of the spirit or the soul itself. The manifestations in question are symptoms, and symptoms only.
Source: Rudolf Steiner – GA 312 – Spiritual Science and Medicine: Lecture XIII – Dornach, 2nd April 1920