With time, scholars of ancient India composed Upanishads, such as the "Pranagnihotra Upanishad", that evolved the focus from external rituals to self-knowledge and to inner rituals within man. The Pranagnihotra is, states Henk Bodewitz, an internalized direct private ritual that substituted external public Agnihotra ritual (a srauta rite). This evolution hinged on the Vedic idea of "devas" (gods) referring to the sense organs within one's body, and that the human body is the temple of Brahman, the metaphysical unchanging reality. This principle is found in many Upanishads, including the Pranagnihotra Upanishad, the Brihadaranyaka Upanishad section 2.2, Kaushitaki Upanishad sections 1.4 and 2.1–2.5, Prasna Upanishad chapter 2, and others. The idea is also found and developed by other minor Upanishads such as the ancient Brahma Upanishad which opens by describing human body as the "divine city of Brahman". Bodewitz states that this reflects the stage in ancient Indian thought where "the self or the person as a totality became central, with the self or soul as the manifestation of the highest principle or god". This evolution marked a shift in spiritual rite from the external to the internal, from public performance through srauta-like rituals to performance in thought through introspection, from gods in nature to gods within. The Śrauta Agnihotra sacrifice thus evolved into Prana-Agnihotra sacrifice concept. Heesterman describes the pranagnihotra sacrifice as one where the practitioner performs the sacrifice with food and his own body as the temple, without any outside help or reciprocity, and this ritual allows the Hindu to "stay in the society while maintaining his independence from it", its simplicity thus marks the "end station of Vedic ritualism".