Vedicism is the oldest recorded religion of India, introduced by the Indo-European immigrants. It venerates a singular cosmic order (rita) above a multitude of gods, who represent individual natural forces (Agni, Vayu, Surya) or aspects of order (Indra, Varuṇa, Mitra). At its core is the sacrifice, which facilitates communication between the sacrificer and the gods, involving a mutual exchange of gifts. The sacrificial rites are performed by priests, who, as Brahmins, constitute the highest class within the social and caste system established by the Vedic religion. The foundations of Vedic religion are found in four collections, of which the Rigveda is the oldest. It contains 1,028 hymns, divided into ten books, representing the "sacred knowledge in verses." The poems were composed in archaic Indo-Aryan (Sanskrit) around 1500-1000 BCE.