Parvati is thus an embodiment of divine knowledge and the mother of the world. She appears as the shakti, or essential power, of the Supreme Brahman. Her primary role is as a mediator who reveals the knowledge of Brahman to the Vedic trinity of Agni, Vayu, and Indra, who were boasting about their recent defeat of a group of demons. But Kinsley notes: "it is little more than conjecture to identify her with the later goddess Satī-Pārvatī, although later texts that extol Śiva and Pārvatī retell the episode in such a way to leave no doubt that it was Śiva's spouse.." Sati-Parvati appears in the epic period (400 BC–400 AD), as both the Ramayana and the Mahabharata present Parvati as Shiva's wife. However, it is not until the plays of Kalidasa (5th-6th centuries) and the Puranas (4th through the 13th centuries) that the myths of Sati-Parvati and Shiva acquire more comprehensive details. Kinsley adds that Parvati may have emerged from legends of non-aryan goddesses that lived in mountains. While the word Umā appears in earlier Upanisads, Hopkins notes that the earliest known explicit use of the name Pārvatī occurs in late Haṃsa Upanishad (Yoga / Shukla Yajurveda).