In occultism there is a saying which can now be made known: In the astral world, every lie is a murder. The full significance of this saying can be appreciated only by someone who has knowledge of the higher worlds. How readily people say: “Oh, that is only a thought or a feeling; it exists only in the soul. To box someone’s ears is wrong, but a bad thought does no harm.” No proverb is more untrue than the one which says: “You don’t have to pay for your thoughts.”
Every thought and every feeling is a reality, and if I let myself think that someone is a bad man or that I don’t like him, then for anyone who can see into the astral world the thought is like an arrow or thunderbolt hurled against the other’s astral body and injuring it as a gunshot would.
I repeat: every thought and every feeling is a reality, and for anyone with astral vision it is often much worse to see someone harbouring bad thoughts about another than to see him inflicting physical harm. When we make this truth known we are not preaching morality but laying a solid foundation for it.
If we speak the truth about our neighbour, we are creating a thought which the seer can recognise by its colour and form, and it will be a thought which gives strength to our neighbour.
Any thought containing truth finds its way to the being whom it concerns and lends him strength and vigour. If I speak lies about him, I pour out a hostile force which destroys and may even kill him. In this way every lie is an act of murder. Every spoken truth creates a life-promoting element; every lie, an element hostile to life. Anyone who knows this will take much greater care to speak the truth and avoid lies than if he is merely preached at and told he must be nice and truthful.
Source: Rudolf Steiner – GA 95 – At the Gates of Spiritual Science – Lecture II: The Three Worlds – Stuttgart, 23rd August 1906