Contains the Ineffable Name. It is the AUM--to be pronounced only mentally, under penalty of death. The Unrevealed Para-Brahma, the Passive-Principle; the absolute and unconditioned "mukta," which cannot enter into the condition of a Creator, as the latter, in order to think, will, and plan, must be bound and conditioned (baddha); hence, in one sense, be a finite being. "THIS (Para-Brahma) was absorbed in the non-being, imperceptible, without any distinct attribute, non-existent for our senses. He was absorbed in his (to us) eternal (to himself) periodical, sleep," for it was one of the "Nights of Brahma."Therefore he is not the First but the Eternal Cause. He is the Soul of Souls, whom no being can comprehend in this state. But "he who studies the secret Mantras and comprehends the Vach" (the Spirit or hidden voice of the Mantras, the active manifestation of the latent Force) will learn to understand him in his "revealed" aspect.
Source: Isis Unveiled Chapter VI