Vaisesika Sūtra 3.2.14

अहमिति प्रत्यगात्मनि भावात्परत्राभावादर्थान्तरप्रत्यक्षः ॥१४॥

aham iti pratyag-ātmani bhāvāt paratra abhāvāt arthāntara-pratyakṣaḥ ॥14॥

[अहम् (aham) - I; इति (iti) - thus; प्रत्यगात्मनि (pratyag-ātmani) - in the inner ātman; भावात् (bhāvāt) - due to presence; परत्र (paratra) - elsewhere; अभावात् (abhāvāt) - due to absence; अर्थान्तर (arthāntara) - of another object; प्रत्यक्षः (pratyakṣaḥ) - direct perception;]

The perception "I" is direct of the inner ātman, as it is present there and absent elsewhere.