Upaniṣads · Chāndogya 7.18

Chāndogya 7.18.1

यदा वै मनुतेऽथ विजानाति नामत्वा विजानाति मत्वैव विजानाति मतिस्त्वेव विजिज्ञासितव्येति । मतिं भगवो विजिज्ञास इति ॥ ७.१८.१ ॥

When one thinks, then one knows. One does not know without thinking; only after thinking does one know. But thought itself must be inquired into. "Venerable sir, I wish to inquire into thought."