The universe appeared to the poets of the Rigveda to be divided into the three domains of earth, air, and heaven, a division perhaps also known to the early Greeks. This is the favourite triad of the Rigveda, constantly mentioned expressly or by implication. The solar phenomena are referred to heaven, while those of lightning, rain, and wind belong to the air. In the three worlds the various gods perform their actions, though they are supposed to dwell only in the third, the home of light. The air is often called a sea, as the abode of the celestial waters, while the great rainless clouds are conceived sometimes as rocks or mountains, sometimes as the castles of demons who war against the gods. The thundering rain-clouds become lowing cows, whose milk is shed and bestows fatness upon the earth. The higher gods of the Rigveda are almost entirely personifications of natural phenomena, such as Sun, Dawn, Fire, Wind. Excepting a few deities surviving from an older period, the gods are, for the most part, more or less clearly connected with their physical foundation. The personifications being therefore but slightly developed, lack definiteness of outline and individuality of character. Moreover, the phenomena themselves which are behind the personifications have few distinctive traits, while they share some attributes with other phenomena belonging to the same domain. Thus Dawn, Sun, Fire have the common features of being luminous, dispelling darkness, appearing in the morning. Hence the character of each god is made up of only a few essential qualities combined with many others which are common to all the gods, such as brilliance, power, beneficence, and wisdom. These common attributes tend to obscure those which are distinctive, because in hymns of prayer and praise the former naturally assume special importance. Again, gods belonging to different departments of nature, but having striking features in common, are apt to grow more like each other. Assimilation of this kind is encouraged by a peculiar practice of the Vedic poets--the invocation of deities in pairs. Such combinations result in attributes peculiar to the one god attaching themselves to the other, even when the latter appears alone. Thus when the Fire-god, invoked by himself, is called a slayer of the demon Vritra, he receives an attribute distinctive of the thunder-god Indra, with whom he is often coupled. The possibility of assigning nearly every power to every god rendered the identification of one deity with another an easy matter. Such identifications are frequent enough in the Rigveda. For example, a poet addressing the fire-god exclaims: "Thou at thy birth, O Agni, art Varuna; when kindled thou becomest Mitra; in thee, O Son of Might, all gods are centred; thou art Indra to the worshipper" (v. 3, 1).