The earnestness with which the Upaniṣads quest for truth is evident in these terse lines from the Bṛhadāraṇyaka Upaniṣad (verse 1.3.28):
From the unreal lead me to the real.
From darkness lead me to light.
From death lead me to immortality.
Argumentation was the preferred means of questing for truth for the Vedic thinkers. When their arguments proved inadequate, they would revise their ideas—but this method has ensured that the material in the Upaniṣads is diverse, unsystematic, and rife with contradictory philosophical opinions. The Upaniṣads cannot be reduced into a single philosophical system.