Goddesses play an important role in the study and practice of Tantra Yoga, and are affirmed to be as central to discerning the nature of reality as are the male deities. Although Parvati is often said to be the recipient and student of Shiva's wisdom in the form of Tantras, it is Kali who seems to dominate much of the Tantric iconography, texts, and rituals. In many sources Kāli is praised as the highest reality or greatest of all deities. The Nirvana-tantra says the gods Brahma, Vishnu, and Shiva all arise from her like bubbles in the sea, ceaselessly arising and passing away, leaving their original source unchanged. The Niruttara-tantra and the Picchila-tantra declare all of Kāli's mantras to be the greatest and the Yogini-tantra, Kamakhya-tantra and the Niruttara-tantra all proclaim Kāli vidyas (manifestations of Mahadevi, or divinity itself). They declare her to be an essence of her own form (svarupa) of the Mahadevi.