I consider it a requirement for every anthroposophy teacher to represent only as much of the teaching as he can justify to the best of his conscience. That is, I require every anthroposophical teacher to say only that of which he has direct knowledge, an immediate knowing. Not one word should be spoken of these higher worlds by the spiritual teacher if he cannot discover it himself, precisely with the same right as no one can speak about chemistry who has not studied it. That is why I shall say only what I can say with absolute certainty in lectures. No one can paint the astral world as a whole; it is richer in content and more voluminous than our physical world. I admit that even the spiritual researcher can be mistaken in details, just as one can be mistaken in the physical world if, for example, one wants to determine the height of a mountain. But just as little as such a mistake in detail can be a reason to deny the physical world, just as little can there be cause for a person to deny the reality of the astral world because of a mistake in a detail.
Source (German): Rudolf Steiner – GA 88 – Über die astrale Welt und das Devachan – Berlin, 28 October 1903 (page 32)
Translated by Nesta Carsten-Krüger