Vaisesika Sūtra 2.1.11

अद्रव्यवत्वेन द्रव्यम् ॥११॥

adravyavatvena dravyam ॥11॥

[adravyavatvena = by the absence of substance-ness; dravyam = is a substance;॥]

(Air) is a substance with the absence of 'substance-ness'.