The Chandyoga volume contains the central doctrine "tat tvam asi," while others advance the "neti-neti" concept through the character of Yajnavalkya [yahj-nah-VALK-yuh].  
A 2003 study by Yohanan Grinshpon claims that Western commentators have misrepresented these books as pure philosophy and argues that they can only be understood as a story of characters. They first emphasized the importance of cosmic knowledge in avoiding punarmrtyu [poo-nahr-murt-yoo], or "repeated death."  The Chandyoga volume contains the central doctrine "tat tvam asi," while others advance the "neti-neti" concept through the character of Yajnavalkya [yahj-nah-VALK-yuh].  Including the Great Forest Texts, they directly answer questions about the sacrifices described in the Rig Veda, and the earliest ones, such as the Isa and Kena, are the basis for the Vedanta school.  FTP, identify these texts with a name meaning "sit down near" which outline the philosophy of Hinduism and were a major influence on Arthur Schopenhauer.
A 2003 study by Yohanan Grinshpon claims that Western commentators have misrepresented these books as pure philosophy and argues that they can only be understood as a story of characters.
Upanishads