The ultimate exhortation of the Upaniṣads is that man should seek to unite his soul or mind with the universal soul, the Brahman (the undivided, timeless, and motionless living principle that is the author of the universe). Here are two verses (verse 2.2.3 and 2.2.4) from the Mundaka Upanisad (translation by Valerie Roebuck):
Seize as your bow the great weapon of the Upaniṣad,
And set in it an arrow sharpened by contemplation.
Draw it with a mind that has attained the nature of that.
The target is imperishable: pierce that.
The OM (pranava) is the bow, the arrow the self:
Brahman is its target, it is said.
It must be pierced by one who is not careless:
So, like the arrow, one will become of a kid with it.