The word veda itself means “knowledge.” The original four Vedas—the Rigveda, the Yajurveda, the Samaveda, the Atharvaveda—dating in their present form back to the early Indo-European period, 1500 to 1000 B.C.E., are poetic Sanskrit works containing stories, hymns, incantations, and rituals for sacrifice. They were not transmitted in writing because oral transmission by rishis (seers, gurus, sages, brahmins) was considered more accurate. Thus, these texts are considered to be particularly sacred, or sruti (“heard”), texts as opposed to less sacred, or smrti (“remembered”), written texts. It is believed by Hindus that the Vedas have always existed and that with the help of a series of beings known as Manus, or with the help of avatars (incarnations of the god Vishnu), the texts re-emerge even after regular dissolutions of the universe. Attached as corollaries to the original Vedas are several groups of texts also considered to be sruti. These texts—Brahmanas, Aranyakas, certain Upanishads, Vedangas, and Sutras—are traditionally included as Vedas.