ثم نطبق الجملتين، بأن نجعل الأول من الجملة الأولى بإزاء الأول من الجملة الثانية، والثاني بالثاني
Then align the collections: first with first, second with second, and so on.
Taṭbīq means a systematic matching of members in two ordered series. The argument asks whether every term has a counterpart or whether one series eventually lacks a corresponding term.
The method produces two alternatives: full correspondence throughout, or a point at which the correspondence ceases. The classical proof assigns a different implication to each case.