prayogaḥ yadanityakṛtaṃ tadanityaṃ yathā ghaṭādi, anityakṛtaś ca varṇakrama iti svabhāvahetuḥ
The application [is]: whatever is created by something non-eternal is non-eternal, like a pot etc., and the sequence of letters is created by something non-eternal - this is a reason based on essential nature