You can imagine what a difficult situation one is in as representative of spiritual science: One not only has the representatives of the religions against one, but also the whole philosophical world and their followers as well. Every Tom Dick or Harry then comes and says: Well, you claim this or that about the spiritual world; but after all, Kant has already proven – so they say – that nothing can be known about it! – This is actually the most generalizing objection that one can make. Someone can say: I do not want to hear anything that Steiner says, because Kant has already proven that nothing can be known about the Spiritual world.
Source (German): Rudolf Steiner – GA 353 – Die Geschichte der Menschheit und die Weltanschauungen der Kulturvölker – Dornach, May 14, 1924 (page 240)
Translated by Nesta Carsten-Krüger