ityevaṃ svavācaiva paro 'bhidhāsyatīti manyamānaḥ prasaṅgasādhanatvam eva tāvad asya prakṛtasya samarthayitum āha tatra cāhur ityādi
Thinking that the opponent would say this through his own words, [the author] states "tatra cāhur" etc. to establish that this is indeed a reductio ad absurdum of the matter at hand