al-Kafi by al-Kulayni is the biggest and most important Shia book of Hadith, it is of the same importance of Sahih al-Bukhari when it comes to the average Muslim. However, the difference between the two books is that al-Bukhari collected the most authentic reports while al-Kulayni filled his Kafi with the weakest narrations making it really hard for the reader to come across any Sahih Hadith in the book.

Of course we all remember what al-Kulayni wrote as the reason for why he compiled his book:
“Verily, you solemnly wished that you possess a book which is sufficient, brings together the entire Islamic sciences of the knowledge of religion within it, wholly satisfies the needs of the student, acts as a reference for the seekers of guidance, and would be used by those who want to attain the knowledge of religion and practice upon it by deriving correct [şaĥīĥ] narrations of the truthful ones (as) and the upright and acted upon traditions from it—through which the compulsory duties of Allāh, the Powerful and Exalted, and the tradition of His Prophet (saws) can be fulfilled.
And you said: ‘If that happens, I can hope that (the book) would be a means through which Allāh will rectify our brothers and people of our religious community through his support and grace, and take them closer to their salvation.’”
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A famous Shia scholar called al-Sayyed ‘Ali abu al-Hassan al-Musawi al-‘Amili replies to Ayatullah al-Khoei who claimed that al-Kulayni has not authenticated his book and has placed in it weak narrations which contradict the Quran:

وليته تأمل قليلاً قبل أن يتفوه بهذه الدعوى الباطلة , وهل يجوز حفظ ما خالف الكتاب عن الضياع , أو ماكان فيه الريب و الموافق للعامة العمياء مع إحراز كونه للتقية بعد الاشتغال عشرين سنة , و الفرض أن مادعى ثقة الإسلام إلى تصنيف كتابه ما قد أشار إليه ؟!فتبا لأثبت علماء الرجال الناقد الخبير , الشيخ النجاشي على افترائه في دعواه : أن الكليني أوثق الناس في الحديث و أثبتهم .و ألف تب لأوثق الناس و أثبتهم , حيث جمع في كتابه بعد أن ضيع عشرين سنة من عمره , جمع فيه الشاذ المخالف للمجمع عليه , وحفظ فيه ما خالف الكتاب من زخرف و باطل , و قد ترك لسائله المتحير و لإخوانه و لأهل ملته ممن لا يفقهون التمييز بين الحديث , ولا يعرفون صحيحه من ضعيفه , وما يجوز أن يعمل به و مالا يجوز أن يستند إليه .

“I wish he(al-Khoei) could have been a bit more patient and observant before he uttered those false words! Is it permissible for him(al-Kulayni) to preserve in it(al-Kafi) that which contradicts the Holy Book? Is it permissible for him to preserve in it that which casts doubts and agrees with the blind sunnies after working on it(al-Kafi) for twenty years? and let us say that Thiqat al-Islam(Kulayni) compiled his book for the reasons he stated.
Then Damn the knowledgeable and expert scholar of RIjal al-Sheikh al-Najashi for saying that: al-Kulayni is the most trustworthy and most knowledgeable of people in Ahadith.
And a Thousand Curses be upon the most trustworthy and most knowledgeable of people in Ahadith(al-Kulayni) because he collected in his book(al-Kafi) after wasting twenty years of his life that which is weak and that which contradicts what is agreed upon and because he collected in it nonsense and falsehood that contradicts the Holy Book. Then he(al-Kulayni) left the confused person who was seeking his help and he left his brothers and his sect who are unable to distinguish between the Ahadith, he left them the Sahih and the Da’eef and that which is permissible to act upon it and that which is not permissible to act upon it.”

Then Abu al-Hassan suggests that al-Kulayni should have wrote in his book:

ثم يقول : وقد يسر الله تأليف كتاب يأخذ منه من يريد علم الدين و العمل به بالآثار الصحيحة و لكنني لم أميّز ذلكو أنا : العالم بالأخبار و العارف بالرجال .و أنا : أوثق الناس في الحديث و أثبتهم .و أنا شيخ أساطين المذهب و فقهائهم العالمين بالحديث .و أنا تلميذ من اجتمعت الكلمة على غاية عملهم بالحديث ومعرفتهم بالرجال , وحفظهم للأخبار , بل تركت مهمة التمييز لك أيها السائل المتحير , و لأخوانك و أهل ملتك , إذ رأيت نفسي أني اعجز عن تهذيب ذلك بعد عشرين سنة , فتركت الامر في ذلك لكم , فإني لا أعرف إلا أقل القليل . و إني اعتذر إليك , فإن مذهب أهل البيت , و الطائفة الحقة , لا يستطيع مثلي و انا من عرفت تصنيف ما سألت عنه .إذ بغاية الأسف أقول : لا يوجد عندنا من الأحاديث الصحيحة إلا النادر و الكتب و الأصول قد ضاعت وفقدت , و العلماءلم ندركهم ولم نعاصرهم , و قد ماتوا و مات العلم معهم “

“Then he says: And Allah has made it possible for me to compile a book which wholly satisfies the needs of the student and acts as a reference for those who wish to act upon the correct narrations but I can not tell the difference between them (Sahih and Da’eef).
And I am: The knowledgeable in the narrations and the conditions of narrators.
And I am: The most trustworthy and most knowledgeable of people in Ahadith.
And I am: The sheikh of the biggest scholars of the Madhab and those who know the Ahadith.
And I am: The student of the ones that are the most knowledgeable in Hadith and Rijal by consensus, but I have left it to you my confused follower and to your brothers and nation to distinguish between the narration (Whether Sahih or Da’eef) because I find my self incapable of doing so after twenty years so I left it all to you because my knowledge is small and limited. I apologise to you because someone like me cannot fulfil your request.


I regrettably say: We do not posses any of the Sahih Ahadith except rarely and the books of Usool have all been lost and we did not meet the big scholars, they died and the knowledge died with them.”

– end –

source: al-Fawaed al-Rijaliyah & Resalat al-Intisar li Sihhat al-Kafi page 113-114.