Hindu Scriptures are classified into: Shruti (meaning "heard"), Smriti (meaning "remembered"), and Nyaya (meaning "logic"). These classifications are based on the origin of the Scripture. Shruti scripture is thought of as something which was heard directly from God, while Smriti scripture refers to what was written down and remembered. Shruti is considered more authoritative since it was obtained straight from God. The Vedas are shruti. The rest of Hindu scripture is known as smriti, except for the Vedanta- sutras which are classified as nyaya. Smriti Scriptures consist of three types: Itihaasa-s - These are epics which were written to make known the human attributes of the Divine. It is comprised of two epics: Ramayana and Mahabharata, which are the stories of two incarnations of Lord Vishnu, Rama and Krishna, respectively. PuraaNa-s - Each story emphasizes morals and is usually a story about a hindu deity fighting for upholding these morals. There are eighteen puraaNa-s and eighteen sub-puranas. Agamas - These are rules for the ritual, rites and worship of Gods. There are five of them. The oldest and most sacred Hindu scriptures, The Vedas, originated around 1500 B.C. Hindus believe that Vedas are timeless. The word Veda means "knowledge." It is believed that this knowledge came directly from Lord Krishna, the Supreme Personality of Godhead. There are four Vedas. They are named Rig, Sama, Yajur, and Atharva. The Vedas consist of prayers to the One Divine and the divinities of nature; for example, the Sun, the Rain, the Wind, and the Fire. The Vedas also include prayers for matrimony, progeny (children), prosperity, concord, domestic rites, formulas for magic, and more. They deal with day-to-day necessities as well as spiritual realization. They are written in metrical verse, usually of three to four lines in length.