prayogaḥ yat sarvasāmarthyaśūnyaṃ tadavastu yathā vandhyāputraḥ sarvasāmarthyaśūnyaṃ cākāśādīti svabhāvahetuḥ
The formal argument is: Whatever is devoid of all potency is non-existent, like the son of a barren woman; and ākāśa etc. are devoid of all potency - this is a natural reason.