In the Brahmanas layer of Vedic literature, Surya appears with Agni (fire god) in the same hymns. Surya is revered for the day, and Agni for its role during the night. According to Kapila Vatsyayan, the concept of a Surya–Agni relationship evolves, and in later literature Surya is described as Agni representing the first principle and the seed of the universe. It is in the Brahmanas layer of the Vedas, and the Upanishads that Surya is explicitly linked to the power of sight, and to visual perception and knowledge. He is then internalized and said to be the eye, as ancient Hindu sages suggested abandonment of external rituals to gods in favor of internal reflection and meditation of the gods within, in one's journey to realize the Atman (soul, self) within, in texts such as the Brihadaranyaka Upanishad, Chandogya Upanishad, Kaushitaki Upanishad, and others.