Physical Reality: J. Muir relates the commentary of Sayana on the three steps of Vishnu mentioned in the White YajurVeda (5.15) as 'Vishnu, becoming incarnate as Trivikrama (the god who stepped thrice), strode over, - i.e. stepped by separate strides over, - this whole [universe]. It is this that [the rishi] expresses [in the worlds], 'In three places he planted his step,' i.e. one step on the earth, a second in the atmosphere, and a third in the sky: in the successive forms of Agni, Vayu, and Surya'. The meaning implied seems to be related to the fact that Agni (fire) is present on the ground, Vayu (air) is present in the atmosphere, and Surya (the sun) is present in the sky or heavens; thus the three steps symbolise the three worlds or planes of physical reality, i.e. the ground, the air, and the sky.