Vaisesika Sūtra 9.1.11

आत्मन्यात्ममनसोः संयोगविशेषादात्म प्रत्यक्षम् ॥११॥

ātmany ātma-manasoḥ saṁyoga-viśeṣāt ātma pratyakṣam ॥11॥

[आत्मनि (ātmani) - in the self; आत्म (ātma) - Ātmā; मनसोः (manasoḥ) - and of the mind; संयोगविशेषात् (saṁyoga-viśeṣāt) - due to special conjunction; आत्म (ātma) - the Ātmā; प्रत्यक्षम् (pratyakṣam) - is perceptible;]

Due to a special conjunction of the Ātmā and the mind within ourselves, the Ātmā is perceptible.