The hymns of the Rigveda being mainly invocations of the gods, their contents are largely mythological. Special interest attaches to this mythology, because it represents an earlier stage of thought than is to be found in any other literature. It is sufficiently primitive to enable us to see clearly the process of personification by which natural phenomena developed into gods. Never observing, in his ordinary life, action or movement not caused by an acting or moving person, the Vedic Indian, like man in a much less advanced state, still refers such occurrences in Nature to personal agents, which to him are inherent in the phenomena. He still looks out upon the workings of Nature with childlike astonishment. One poet asks why the sun does not fall from the sky; another wonders where the stars go by day; while a third marvels that the waters of all rivers constantly flowing into it never fill the ocean. The unvarying regularity of sun and moon, and the unfailing recurrence of the dawn, however, suggested to these ancient singers the idea of the unchanging order that prevails in Nature. The notion of this general law, recognised under the name rita (properly the "course" of things), we find in the Rigveda extended first to the fixed rules of the sacrifice (rite), and then to those of morality (right). Though the mythological phase presented by the Rigveda is comparatively primitive, it yet contains many conceptions inherited from previous ages. The parallels of the Avesta show that several of the Vedic deities go back to the time when the ancestors of Persians and Indians were still one people. Among these may be mentioned Yama, god of the dead, identical with Yima, ruler of paradise, and especially Mitra, the cult of whose Persian counterpart, Mithra, obtained from 200-400 A.D. a world-wide diffusion in the Roman Empire, and came nearer to monotheism than the cult of any other god in paganism. Various religious practices can also be traced back to that early age, such as the worship of fire and the cult of the plant Soma (the Avestan Haoma). The veneration of the cow, too, dates from that time. A religious hymn poetry must have existed even then, for stanzas of four eleven-syllable (the Vedic trishtubh) and of four or three eight-syllable lines (anushtubh and gayatri) were already known, as is proved by the agreement of the Avesta with the Rigveda. From the still earlier Indo-European period had come down the general conception of "god" (deva-s, Lat. deu-s) and that of heaven as a divine father (Dyaus pita, Gr. Zeus pater, Lat. Jupiter). Probably from an even remoter antiquity is derived the notion of heaven and earth as primeval and universal parents, as well as many magical beliefs.