If we were now able to develop as strong a consciousness of what happens to us during sleep as, under certain circumstances, men of bye-gone times could do, we should never think of doubting the existence of the spirit. We should then be able to remember not only subconsciously, but in full consciousness, what we encounter during our sleep. If a man were to experience in full consciousness what he passes through in sleep, it would be just as absurd for him to deny the existence of spirit as it would be for a waking man to deny the fact that there were tables and chairs.
Source: Rudolf Steiner – GA 175 – Cosmic and Human Metamorphoses – Lecture 5 – The human soul and the universe – Berlin, 6th March 1917
Translated by Harry Collison