These refer to four hundred books which have been labelled with the title Asl (principal) due to them being principal sources to which their scholars refer and upon which they rely.
Al Tahrani mentions from his teacher al Mufid:
صنفت الإمامية من عهد أمير المؤمنين عليه السلام إلى عصر أبي محمد الحسن العسكري عليه السلام أربعمائة كتاب تسمى الأصول، وهذا معنى قولهم له أصل
The Imamiyyah authored from the era of Amir al Mu’minin ‘alayh al Salam up to the era of Abu Muhammad al Hassan al ‘Askari four hundred books which are dubbed the Usul, principal sources. And this is what their dubbing of them as Asl means.
Any hadith book whose author has heard the narrations compiled in his book from the Infallible Imam directly, or from someone who heard from the Imam directly, is an Asl. This is due to it being the first record of those narrations and the reference point to which later people have recourse.
Al Tahrani says after a lengthy discussion regarding the meaning of Asl:
فالأصل من كتب الحديث هو ما كان المكتوب فيه مسموعا لمؤلفه عن المعصوم، أو عمن سمع منه لا منقولا عن مكتوب فإنه فرع منه
So an Asl in the books of hadith is a book the recorded narrations of which are directly heard by its author from the infallible, or from the person who heard from him. They are not cited from another record, for in that case it would a secondary source.
And the author of the book Dirasah Hawl al Usul al Arba’ Mi’ah mentions:
بلغ الرواة عنه أربعة آلاف رجل، وانصرفت طائفة كبيرة من هؤلاء لضبط ما رووه عن الإمام سماعا في كتاب خاص في مواضيع الفقه، والتفسير والعقائد وغيرها، وقد اصطلح التاريخ الشيعي على تسمية هذه الكتب بالأصول، كما حصرها في أربعمائة أصل، وهذا ما نعنيه بالأصول الأربعمائة
The narrators from him, Jafar al Sadiq, are four thousand. And a large group of these narrators have documented what they narrated directly from the Imam regarding various topics of jurisprudence, exegesis, beliefs, etc., in special books. The Shia history has concurred on naming these books ‘the Usul’ and has confined them to four hundred principal works. This is what we mean by the four hundred Usul.
The author immediately after this has discussed the difference of opinion regarding the definition of Asl. Thereafter, he has enlisted the names of the authors of these Usul, but their amount does not even reach eighty. Thereafter, he presents a general study of these Usul which he then follows up with a more specific study of the Usul which exist in their entirety, or parts of which exist; and these are only twenty-eight. In this study he has reached the following conclusion. He says:
In this study we have reached the following conclusions:
Firstly: The Asl is something the Shia scholars coined as terminology in the fifth century A.H.
Secondly: The scholars of hadith have proposed various views in delineating the definition of al Asl, most of which are based on sheer conjecture and assumption, as is stated by al Sayyid Muhsin al Amin.
The word ‘al Asl’ has two meanings: The first is the technical meaning which refers to a collection of hadith which is narrated directly from Imam al Sadiq in most instances and is compiled by his transmitters. We have supported this with the statements of the early scholars and we have stated that most of those enlisted by al Tusi and al Najashi as the authors of the Usul are the companions of Imam al Sadiq ‘alayh al Salam; and this is also according to the present study of the Usul.
The second is the linguistic meaning which means source or reference, as per its usage in our time. This is because it is used to refer to books of sciences other than hadith, or it was used before the fifth century A.H.
Thirdly: Specifying the era of compilation to the era of Imam Jafar al Sadiq, i.e. those who narrated from him. But this contradicts the fact that some of them also narrate from his father, al Baqir, or his son, al Kazim ‘alayh al Salam.
Fourthly: If by way of the word Asl the literal meaning is intended, then the Usul of the Shia are 6600 approximately. And if the technical meaning is intended then they will not be more than a hundred, and those enlisted in the two Fihrists of al Tusi and al Najashi are only seventy and some odd Usul.
Fifthly: The Usul themselves have been discarded due to the four early books and the collections of hadith comprising of these Usul and other Shia sources. This is why the hadith scholars did not require the copies of the Usul themselves, i.e. due to their content and narrations being present in these books which were compiled in an era subsequent to the era of the compilation of the Usul. Thus, after my extensive search for the Usul enlisted by al Tusi, I did not come across more than three Usul which exist today. Likewise, from the books which are described as Usul I have come across more than twenty-seven books, probably I will be fortunate to come across more of them in the future.
And al Shahid al Thani says in this regard:
كان قد استقر أمر الإمامية على أربعمائة مصنف سموها أصولاً فكان عليها اعتمادهم، وتداعت الحال إلى أن ذهب معظم تلك الأصول ولخصها جماعة في كتب خاصة تقريبا على المتناول، وأحسن ما جمع منها (الكافي) و(التهذيب) و(الاستبصار) و(من لا يحضره الفقيه)
The matter of the Imamiyyah has settled upon four hundred books which they dub the Usul. Hence upon them was their reliance. The conditions prompted that most of these Usul vanish. Hence, a group of scholars condensed them in special books in order to facilitate easy access to them. The best books which have been compiled by them are al Kafi, al Tahdhib, al Istibsar, and Man la Yahduruhu al Faqih. [End quote]
Professor ‘Ali al Salus commenting upon this says:
If his statement is correct, it entails that the documentation of opinions and jurisprudential extractions took place alongside the documentation of the narrations narrated from Imam al Sadiq. And the Shia aver that whatever emerged from him is considered to be from the Sunnah, but Imam al Sadiq could not possibly have considered himself to be infallible or as having the right of legislation. Likewise in his time, with the exception of the extremist, there were very few people who considered him to be infallible.
Hence, whatever is narrated from Imam al Sadiq is not in any way different from that which is narrated from the four Imams, viz. Abu Hanifah, Malik, al Shafi’i, and Ahmed, and other luminary scholars besides them. The only difference we notice between them is the differing views which exists between these scholars of Ijtihad. As for the narrations documented in the era of Imam al Sadiq falsely against him, these forgeries cannot exceed his time and the times that passed before him, i.e. it is not possible that they have any connection with those who come after him due to that being considered from the knowledge of the unseen.
Hence, we are not in need of studying these Usul or investigating who their writers were. But what we are concerned with is that which the Twelver Shia exclusively claim has not surfaced so that it be recorded. Therefore, I was very surprised by the title of one of these principles which the author has mentioned on p. 48 and reads as follows: ‘The extract of narrations regarding the twelve Imams’. It is impossible that this type of title be coined in the era of Imam al Sadiq, for there was no one in his time who had knowledge of the names of those who were going to succeed him, because only Allah subhanahu wa ta ‘ala has knowledge of the unseen. Yes, this title could have been coined after the eleventh Imam and thereafter the forged narrations could have been falsely attributed to al Sadiq. This is the reality of what possibly happened. So if this title is attributed to those who lived during the era of al Sadiq, then this entails that the forger of this title has forged narrations against those who lived in the era of the Imam.
Nonetheless, after reading the title the following appeared: ‘The compilation of Ahmed ibn Muhammad ibn ‘Ayyash al Jawhari who died in 401 A.H.’ This clearly shows that the author lived after the eleventh Imam, in fact, between their demises is a gap of a century and a half. So this is what is more in sync with what has previously been stated.
Al Tahrani has enlisted close to 130 Usul in his book al Dhari’ah. Hereunder are some of them:
These Usul are not existent now; however, they are scattered in their reliable books.
Abu Jafar al Tusi, known as ‘Sheikh al Ta’ifah’ states:
إن كثيرا من مصنفي أصحابنا وأصحاب الأصول ينتحلون المذاهب الفاسدة، وإن كانت كتبهم معتمدة
Many authors from our comrades and the authors of the Usul are affiliated to false creeds, even though their books are reliable.
And the author of Miqbas al Hidayah has made clear the ignorance which exists regarding many of the authors of the Usul saying:
ذكر الطوسي في مقدمة فهرسه بأنه سيبين في كتابه أحوال أصحاب الأصول بانتحالهم مذاهب فاسدة، ولم يف بوعده
Al Tusi has stated in the introduction to his al Fihrist that he will state in his book the biographies of the authors of the Usul due to them being affiliated to false creeds, but he did not fulfil his promise.
And an example of the ignorance about the authors of the Usul is the following statement of al Tahrani under the biography of Abu ‘Abdullah ibn Hammad al Ansari:
ولم يذكر في كتب الرجال ترجمة أبي عبد الله بن حماد
And in the books of transmitter-biographies the biography of Abu ‘Abdullah ibn Hammad has not been mentioned.
Also, an example of the low stature of the Asl is the following statement of al Tahrani in al Dhari’ah under the biography of Ahmed ibn ‘Umar al Hallal:
أحمد بن عمر الحلال بياع الحل وهو الشيرج-دهن السمسم- عده الشيخ الطوسي في رجاله من أصحاب الرضا عليه السلام، وقال كوفي أنماطي ثقة، ردي الأصل
Ahmed ibn ‘Umar al Hallal, the merchant of sesame seed oil. Al Tusi has enumerated him in the students of al Rida ‘alayh al Salam and has said, “A reliable narrator from Kufah, from Anmat, who has an Asl of low stature.”
Commenting on this al Tahrani says:
يعني لا يعتمد على أصله لاشتماله على ما يشينه من تصحيف أو غلط أو تغييرات وغير ذلك
I.e. his Asl cannot be relied upon, due to it comprising of that which spoils it, like distortions, mistakes, and changes, etc.
 Al Dhari’ah, 2/130.
 Ibid., 2/126.
 Dirasah, p. 7, citing from the book Ma’ al Ithnay ‘Ashariyyah fi al Usul wa al Furu’ of Professor ‘Ali al Salus, 3/105.
 Ma’ al Ithnay ‘Ashariyyah fi al Usul wa al Furu’, 3/106.
 Al Dhari’ah, 2/135-138.
 Al Tusi: al Fihrist, p. 32.
 Miqbas al Hidayah, 3/27.
 Al Dhari’ah, 2/138.
 Rijal al Tusi, p. 352; entry no. 5213.
 Al Dhari’ah, 2/139.