The Upanishads describe the universe, and the human experience, as an interplay of Purusha (the eternal, unchanging principles, consciousness) and Prakṛti (the temporary, changing material world, nature). The former manifests itself as Ātman (Soul, Self), and the latter as Māyā. The Upanishads refer to the knowledge of Atman as "true knowledge" ("Vidya"), and the knowledge of Maya as "not true knowledge" ("Avidya", Nescience, lack of awareness, lack of true knowledge). Brihadaranyaka Upanishad, states Ben-Ami Scharfstein, describes Maya as "the tendency to imagine something where it does not exist, for example, atman with the body". To the Upanishads, knowledge includes empirical knowledge and spiritual knowledge, complete knowing necessarily includes understanding the hidden principles that work, the realization of the soul of things.