Vaisesika Sūtra 2.1.12

क्रियावत्त्वाद् गुणत्त्ववाच्च ॥१२॥

kriyāvattvād guṇattvavācca ॥12॥

[kriyāvattvād = from the action-ness; guṇattvavācca = from the quality-ness as well;]

Based on action-ness and quality-ness, (Air is a substance).