Question

the main philosophy of the Upanishads

Answer 1

Upanishads are one of world's great repositories of spiritual wisdom.  

Composed verbally by Indian sages during 9th century b.c. as they have attracted attention of scholars and spiritual seekers worldwide.  

They are experiential and at times mystical understanding of cosmos, divine and human self which over centuries become obsolate.  

Maximum Hindu thought self-consciously sees as development of Upanishadic teaching which is considered as shruti divinely.