It is so often said in everyday life: Thoughts are duty-free! — meaning that we can think what we like and nobody in the external world will be affected. This is one important point where a man who has really grasped spiritual impulses is at variance with the materialistic thinker.
The materialist agrees that injury is caused if he throws a stone at a man, but he thinks that a thought of hatred which he may harbour against a fellow-creature, does not hurt him. Those however who have real knowledge of the world know that far, far stronger effects proceed from a thought filled with hatred than can ever be caused by a stone. Everything that a man thinks and feels has its effects in the outer world and the seer can follow with great precision the effect of a loving thought that goes out to another man, and the very different effect that is produced by a thought filled with hatred. When you send out a loving thought to someone the seer perceives a form of light shaped like a sort of flower-calyx, playing around his etheric and astral bodies, thereby contributing something to his vitality and happiness. On the other hand a thought of hatred bores its way into the etheric and astral bodies like a wounding arrow.
Source: Rudolf Steiner – GA 99 – Theosophy of the Rosicrucian: VI: The Law of Destiny
– Munich, 30th May 1907
Translated by M. Cotterell & D.S. Osmond