In the later Vedic works called the Brahmanas and the Aranyakas written mostly in prose, followed the Vedic hymns. There are six systems of orthodox Hindu thought. They are Samkhya, the Yoga, the Nyaya, the Vaisesika, the Mimamsa (Purva mimamsa) and Vedanta (Uttar Mimamsa). Samkhya and Yoga are different schools of one system. The Vaisesika and the Nyaya in later times became so mixed up that initailly Vaisesika had similarity with Mimamsa but it became so mixed with Nyaya that they have been treated together. In addition to these systems some theistic systems began to grow prominent from the ninth century AD. They had their early beginning at the time of the Upanisads.

One of their most important early works is the Bhagavadgeeta. This is rightly regarded as one of the greatest masterpieces of Hindu thought. It is written in verse, and deals with moral, religious and metaphysical problems. In a loose form its lack of system and method gives it its peculiar charm more akin to the poetry of the Upanisads. From the ninth century onwards attempts were made to supplement these loose theistic ideas.The Samhitas are collection of verses ('sam' means together, 'hita' means to put). There are four collections or Samhitas namely Rig veda, Sama veda, Yajur Veda and Atharva Veda.

The Brahmanas were theological treatise written in prose. They explain sacred significance of the different rituals. These works are full of dogmatic assertions, fanciful symbolism and speculations of an unbounded imagination in the field of sacrificial details. The period is believed to be 500 BC.

Aryanakas or forest treatise wherein intelligent people the ritualistic ideas began to give away and philosophic speculations about the nature of the truth was gradually substituted. Free thinking tried gradually to come out of ritualism paved way for Upanishads.

There are more than 100 Upanishads. Some of them were written at later times, as late as fourteenth century. The Upanishads are also known as Vedanta as they are believed to be the later portions of the Vedas. They retained their Vedic names and lineage . The Upanisads attached to the Brahmanas of the Aitareya are called as Aitareya Upanisads.

All these literatures were looked upon as so holy that in the early times that it was thought almost a sacrilege to write them. Hence they were propagated by memorising by heart and mouth to mouth. Hence it was a knowledge that is heard. Hence they were called as Sruti.