The Chandogya Upaniṣad has an account of a conversation between Narada and Sanatkumara—they discuss several philosophical and religious problems, including the problem of difference between mind and will. Here’s a brief excerpt:
Narada: “Blessed one, is there anything greater than mind.”
Sanatkumara: “There is something greater than mind.”
Narada: “Tell me about it, blessed one.”
Sanatkumara: “Will (samkalpa) is greater than mind. When one wills (samkalpayate), one thinks; then one utters speech-one utters it as names. In name the mantras become one, and in the mantras actions become one.
“These have will as their sole end, will as their self, and are established on will. Sky and earth have been formed (sam-klp-); air and space have been formed; the waters and heat have been formed, and rain is formed according to their will (samklpti). Food is formed according to the will of rain. The breaths are formed according to the will of food. The mantras are formed according to the will of the breaths. Actions are formed according to the will of the mantras. The world is formed according to the will of the actions. Everything is formed according to the will of the world. This is will. Worship will.”
(Translation by Valerie Roebuck)