"..we see all religions at their highest point end in mysticism and mysteries, that is to say, in darkness and veiled obscurity. These really indicate merely a blank spot for knowledge, the point where all knowledge necessarily ceases. Hence for thought this can be expressed only by negations, but for sense-perception it is indicated by symbolical signs, in temples by dim light and silence, in Brahmanism even by the required suspension of all thought and perception for the purpose of entering into the deepest communion with one's own self, by mentally uttering the mysterious Om. In the widest sense, mysticism is every guidance to the immediate awareness of that which is not reached by either perception or conception, or generally by any knowledge. The mystic is opposed to the philosopher by the fact that he begins from within, whereas the philosopher begins from without. The mystic starts from his inner, positive, individual experience, in which he finds himself as the eternal and only being, and so on. But nothing of this is communicable except the assertions that we have to accept on his word; consequently he is unable to convince."
-Schopenhauer, The World is Will and Representation, volume 2, chapter XLVIII