The oldest literary texts preserved in any Indo-European language are the Vedas or sacred books of India. These fall into four groups, the earliest of which, the Rig-veda, is a collection of about a thousand hymns, and the latest, the Atharva-veda, a body of incantations and ritual formulas connected with many kinds of current religious practice. These books form the basis of Brahman philosophy and for a long time were preserved by oral transmission by the priests before being committed to writing. It is therefore difftcult to assign definite dates to them, but the oldest apparently go back to nearly 1500 B.C. The language in which they are written is known as Sanskrit, or to distinguish it from a later form of the language, Vedic Sanskrit. This language is also found in certain prose writings containing directions for the ritual, theological commentary, and the like (the Brahmanas), meditations for the use of recluses (the Aranyakas), philosophical speculations (the Upanishads), and rules concerning various aspects of religious and private life (the Sutras).