In all cases where people reject what they hear through Spiritual Science, an instinct of self-preservation is at work; they know that they are incapable of doing the necessary exercises — that is, of practising asceticism in the true sense. A person prompted by the instinct for self-preservation will then say to himself: If these things were to permeate my spiritual life, they would confuse it; I could make nothing of them and therefore I reject them. So it is with a materialistic outlook which refuses to go a step beyond the doctrines of a science it believes to be firmly founded on facts.
Source: GA 58 – Metamorphoses of the Soul, Vol 1: Lecture 6: Asceticism and Illness – Berlin, 11th November 1909
Translated by Charles Davy and Christian von Arnim
The great Rudolf Steiner Quotes Site 