Parallel with the occult training, there is a definite development of thinking, without which no occult training can be carried through. This is because there is something about thinking which does not apply to anything else. When we are on the physical plane, we perceive with the physical senses only what is to be found on that plane. Astral perceptions are valid for the astral plane; devachanic hearing is valid only in Devachan. Thus each plane has its own specific form of perception. But one activity — logical thinking — goes through all worlds. Logic is the same on all three planes. Thus on the physical plane you can learn something which is valid also for the higher planes.
Source: Rudolf Steiner – GA 95 – At the Gates of Spiritual Science – Lecture XII: Occult Development – Stuttgart, 2nd September 1906
Translated by E. H. Goddard & Charles Davy