In the late vedic period, Brahma, the creator god, became the principal god of the vedic pantheon, replacing Indra. In the Upanishad tradition, the personal god Brahma becomes the impersonal creative principle, Brahman. Through mystical experience, these "forest dwellers," men who had withdrawn from domestic life, arrived at the conclusion that Brahman is not the creator of Reality but is Reality itself in all its multiplicity. It is the sacred power of all things, the source of all things, the essence of all things. Brahman is a generative principle: all things come from Brahman. More than this, however, all things are Brahman, for ultimately there is no plurality, but only unity: "what is" is ultimately Brahman in his appearance as plurality. In other words, what exists is Brahman, which permeates all things and in fact is all things, even material things. Brahman is also described as God, Spirit, the Self (atman) and the sacred syllable of Om. Actually, Brahman is ineffable, being beyond description. One cannot define or describe Brahman, because words differentiate and Brahman is beyond all such differentiations. Yet, much is said about Brahman in the Upanishads, so that one must always take what is said as provisional and not fully true.