The oldest Upanishads are in part included in these forest texts, and it is often difficult to draw the line between the Aranyakas and the Upanishads. These texts formed, in most senses than one, the Vedanta, i.e. 'the end of the Vedas.' Most of these texts are of later origin, and chronologically into the end of the Vedic period. Furthermore, it must be remembered that the whole of this Vedic Literature does not only consist of written books, but was only transmitted through word of mouth.
The Aranyakas contrast with the Grhyasutras, treatises intended for domestic life. The Aranyakas discuss philosophy and sacrifice and also provide us with mystic teachings rather than rules for the performance of sacrifices and explanations of ceremonies. The Aranyakas constitute mystical interpretations of the themes presented in the Vedas, unlike the Brahmanas which are concerned with the proper performance of rituals .The Aranyakas were initially secret or hidden teachings but not in the sense of being forbidden or restricted and was primarily conveyed individually from teacher to student. The forest books are as follows:
There are five chapters in this book and each one is considered as a full Aranyaka. The first one deals with the regimen known as Mahaavrata. The explanations are both ritualistic as well as absolutistic. The second one has six chapters of which the first three are about Prana Vidyaa (Prana meaning the vital air). It is in this portion of the Aranyaka that one finds specific statements about how one who follows the Vedic injunctions and performs the sacrifices goes to become the God of Fire, or the Sun or Air and how one who transgresses the Vedic prescriptions is born as lower level beings, namely, as birds and reptiles. Pranaa is Vishwaamitra because the "Vishwa" (universe) is the object of experience of this Praana deity. Vamadeva and Vashishhta are also Praana. The fourth, fifth and sixth chapters of this second Aranyaka constitute what is known as Aitareya Upanishad. The third Aranyaka in this chain ofAranyakas is known as 'Samhitopanishad'. This explains the various ways of reciting the Vedas - like Pada Paatha, Krama Paatha, etc. The fourth and the fifth Aranyaka are technical and dwell respectively on the mantras known as 'Mahaanaamni' and the Yajna known as 'Madhyandina'.
There are ten chapters in this Aranyaka. Chapters one to six form the Aranyaka proper. The first one is the famous Surya Namaskara chapter. The second one is a description of the five Maha Yajnas that every Brahmin has to do daily. The third and fourth chapters go into further technicalities of several other Homas and Yajnas. The fifth is an academic treatise on Yajnas. The sixth one is a collection of 'Pitr-Medha' mantras, that is, the mantras recited on the occasion of, and used for, the rituals for the disposal of the dead body. The 7th, 8th and 9th constitute the well-known Taittiriya Upanishad. The tenth is a long Upanishad known as Maha-Narayana-Upanishad. It contains several important mantras taken from the three Vedas.
There are fifteen chapters in the Shankhayana Aranyaka. The first two chapters deal with the Mahavrata. From the third to the sixth it constitutes the Kausitaki Upanishad. The seventh and eighth are known as a Samhitopanishad. The ninth one talks about the greatness of Prana. The tenth chapter deals with the significance of the Agnihotra ritual. The eleventh chapter prescribes several antidotes in the form of rituals for warding off death and sickness. It also details the effects of dreams. The fruits of prayer are elaborated in the twelfth chapter and the thirteenth chapter gets into more philosophical matters and talks about the disciplines of penance, faith, self control etc. The fourteenth chapter gives two mantras. The first one says "I am Brahman" and the second one declares that one who does not know the meaning of the mantra but still recites it is like an animal who does not know the value that it carries. The last chapter gives a list of genealogy of spiritual leaders from Brahma down to Guna Sankhayana.
This is a famous Upanishad .It is associated with the Shukla Yajur Veda and the self is the subject of discussion here.
The Aranyakas contain little of the exalted mysticism of the Upanishad and is primarily concerned with the same theme as the Brahmanas .They are of interest and of importance primarily to specialists.