Chāndogya Upaniṣad 7.5

1. ‘Thought (_citta_), assuredly, is more than Conception. Verily, when one thinks, then he forms a conception, then he has in Mind, then he utters Speech, and he utters it in Name. The sacred sayings (_mantra_) are included in Name; and sacred works in the sacred sayings.

2. Verily, these things have Thought as their union-point, have Thought as their soul, are established on Thought. Therefore, even if one who knows much is without Thought, people say of him: “He is not anybody, whatever he knows! Verily, if he did know, he would not be so without Thought!” On the other hand, if one who knows little possesses Thought, people are desirous of listening to him. Truly, indeed, Thought is the union-point, Thought is the soul (_ātman_), Thought is the support of these things. Reverence Thought.

3. He who reverences Thought as Brahma—he, verily, attains the Thought-worlds; himself being enduring, the enduring worlds; himself being established, the established worlds; himself being unwavering, the unwavering worlds. As far as Thought goes, so far he has unlimited freedom, he who reverences Thought as Brahma.’ ‘Is there, Sir, more than Thought?’ ‘There is, assuredly, more than Thought.’ ‘Do you, Sir, tell me it.’