Eesha.

The early Upanishads were formulated from between 1000 BC to 300 BC. The ancient and genuine Upanishads are rather few namely Eesha, Kena, Katha, Prashna, Mundaka, Mandukya, Taittiriya, Aitereya, Chandogya Brihadaranyaka Kausitaki Maitrhryaniya and Shvetaaeshvatara.
We know nothing except the names of the sages and thinkers of these Upanishads, they cared so little for personal distinctions that they studiously omitted their names. They chose rather one of the Gods of the Vedas  like Indra and Narada on whom to foster their thoughts. A few like Yajnavalkya, Gargi, Maitreya do find mention that too only their names.
The Upanishads are mostly in the form of dialogue which contain much that is poetry as they were meant to be taught orally, they are pith yin and concentrated, this thus makes their understanding rather difficult but they are compensated by way of comparison and illustrations and the imaginative insights. The thoughts and ideas are eternal in their truths and beauty being in the end of the Vedas they contain the crystallized ideas  which were only hints in the former. The change of emphasis from the Vedas to the Upanishads is quiet evident and logical. All intellectual developments proceed from the material to the spiritual.

The Quest: What lies behind human life, its working its aspirations its desires.
When 5 years old Naciketa redeems his final boon he asks Yama, The God of death:
What lies beyond this transient world? Where does the soul of the dead one reailly go?