وَهُوَ بِالضَّرُورَةِ مُوجِبٌ لِلْعِلْمِ الضَّرُورِيِّ
It necessarily yields necessary knowledge.
Necessary knowledge here means knowledge whose acquisition does not depend on first arranging a formal inferential proof. When widespread reporting satisfies the appropriate conditions, certainty can arise without the listener constructing a separate syllogism.
A person's knowledge that a widely attested city exists need not come from a technical argument laid out premise by premise. Rather, the interlocking public evidence supplies assurance directly. The designation “necessary” does not mean causeless or unconditioned; it means that a fresh theoretical inference is not required for the cognition in question.