The works of Ādi Śaṅkarācārya

Chāndogyopaniṣadbhāṣyam

Bhashya

Bhashya

यदा वै मनुत इति ।मतिः मननं तर्कः ॥

In plain words

As for the expression 'when indeed one reflects'—mati means pondering and logical reasoning.

Close to the Sanskrit

'When indeed he reflects'—mati is pondering, reasoning.