Yoni (Sanskrit: योनि), states Monier Monier-Williams, appears in the "Rigveda" and other Vedic literature in the sense of feminine life-creating regenerative and reproductive organs, as well as in the sense of "source, origin, fountain, place of birth, womb, nest, abode, fire pit of incubation". Other contextual meanings of the term include "race, caste, family, fertility symbol, grain or seed". It is a spiritual metaphor and icon in Hinduism for the origin and the feminine regenerative powers in the nature of existence. The "Brahma Sutras" metaphorically calls the metaphysical concept "Brahman" as the "yoni of the universe", which Adi Shankara states in his commentaries means the material cause and "source of the universe".