How is it known that a Hadith concerning a particular matter has not been reported, if it is said that "there is no Hadith" or "it has no basis"?

We have not come across a definitive number for the authentic (sahih (definition)) hadiths because it is difficult to specify a certain number for them. Authentication (tashih) and weakening (tad'if) are relative; a hadith (definition) might be considered authentic by some and weak by others. Imam Al-Shafi'i was of the opinion that knowledge of the Sunnah (definition) cannot be gathered by one man alone, but rather it is gathered by the scholars of the Ummah amongst themselves, as they are at varying levels in their comprehension of it. When a scholar, mufti (definition), or hadith narrator denies the existence of a hadith regarding a certain matter, he is only denying his knowledge of it, not denying its absolute existence, unless the hadith is something that is impossible to have been issued by the Prophet, peace and blessings be upon him.