This notion was confirmed by al-Iraqi in exposition of his Alfiyyah...

This notion was confirmed by al-Iraqi in exposition of his Alfiyyah, when saying: Whenever men of hadith say: This hadith is sahih, they intend to say — as it appeared to us out of the externals of isnad — that its veracity is decisive in itself because of possibility of error and forgetfulness on the part of the thiqah (trustworthy).

In al-Qawati; al- Sam'ani writes: The sahih tradition cannot be recognized only through the narration of trustworthy men but also through comprehension, knowledge, intensive hearing and study. Al-Hakim says: So many a hadith are there whose isnad contains only one reliable thiqah narrator, and so they be weak and defective. Abd al-Rahman ibn Mahdi says: Recognizing the (correct) hadith is an inspiration! and when asking that who is expert in defects of hadith: Wherefrom is this?

he will have no hujjah to argue with. These were some of the statements of the ulama’ about the hadith which they made sahih, so how would be the case with the traditions that were reported in their books and were considered sahih by them? And after all this what is to do by that who intends to recognize the correct hadith in which the heart feels assured and the self is confident?

And which way has he to adopt so as to be guided to distinguish it from among other traditions, while facing such and other sayings that cause perplexity and raise doubt and suspicion? Which one has he to adopt and which one has he to forsake?

Especially after being aware of what Ibn al-Salah quoted in his Fatawa from the Imams (leaders) of hadith including their statements about forms of hadith, when saying: The imams classified hadith into the following forms: 1- a hadith with incorrect isnad and matn (text). 2- or a hadith with incorrect isnad and sahih text. 3- or a hadith with unknown isnad and matn. 4- or a hadith with correct isnad and matn. 5- or a hadith with weak isnad and matn.

These were the five forms of hadith among whose ambiguous and bewildering ravines the researcher may go astray, knowing no outlet toward deliverance, because they (traditionists) have neither manifested their boundaries nor singled out between their sorts, nor laid down criteria for evaluating them so as people be acquainted with them.