Apa yang membantu al-Shafi'i dalam meletakkan asas ilmu usul ialah kenyataan bahawa beliau hidup pada zaman ketika ilmu fiqh Islam telah mula berkembang pesat. Di negeri-negeri Islam, kumpulan-kumpulan mujtahidin mula terbentuk dan mereka mula membentuk mazhab-mazhab. Perdebatan antara para mujtahidin dan para pendukung mazhab-mazhab mengambil pelbagai sudut pandangan dalam fiqh dan dalil-dalilnya. Maka beliau terlibat dalam perdebatan dengan mereka yang berhujah dalam perbincangan tersebut, dan perbincangan inilah yang membimbing beliau untuk memikirkan kaedah-kaedah umum dan menyeluruh sebagai ukuran pengawalseliaan yang sepatutnya menjadi asas kajian dan istinbat. Beliau menghimpunkan kaedah-kaedah ini sebagai satu badan ilmu, iaitu ilmu usul al-fiqh.
Perkara yang mengagumkan tentang usul al-Shafi'i ialah beliau membincangkan usul dengan cara perundangan dan bukan dengan cara logik. Salah satu bahaya terbesar bagi kajian, bahkan bagi kebangkitan Ummah khususnya dalam fiqh dan usul, ialah jalan logik. Al-Shafi'i dengan jelas menjauhkan diri daripada jalan logik dan berpegang kepada jalan perundangan. Beliau tidak berminat dengan kaedah-kaedah teori atau andaian. Beliau mahu mengawal perkara-perkara yang nyata dan sedia ada, iaitu beliau mengambil nas-nas Syariah dan berhenti pada batas nas tersebut serta batas realiti yang ditunjukkan oleh nas itu dan yang disaksikan sendiri oleh manusia. Mengenai persoalan nasakh dan mansukh, beliau menetapkan kaedah-kaedah nasakh daripada perkara-perkara yang, pada pandangan beliau, telah terbukti mengandungi nasakh, sama ada yang disebut dalam ayat atau hadith itu sendiri, atau daripada petunjuk (dalalah) nasakh, atau apa yang diriwayatkan tentang Rasulullah ﷺ dalam bentuk hadith yang menunjukkan nasakh, atau apa jua yang dilaporkan tentang para Sahabat Rasulullah ﷺ dalam bentuk riwayat dan keputusan hukum. Tidak seperti ramai yang datang selepasnya, yang apabila melihat pertentangan antara dua ayat atau dua hadith, terus mendakwa bahawa salah satunya telah dimansukhkan oleh yang lain, sehingga mereka akhirnya melakukan kesilapan yang sangat teruk. Apabila al-Shafi'i mengemukakan sesuatu kaedah, beliau tidak mengambilnya daripada premis logik (muqaddimah mantiqiyyah), sebaliknya beliau menunjukkan sumber-sumber yang darinya kaedah itu diambil, sama ada daripada suatu riwayat tentang Nabi ﷺ atau daripada fatwa-fatwa para Sahabat. Pendekatannya dalam merumuskan kaedah-kaedah perundangan (qawa'id dabitah) adalah pendekatan praktikal, yang bergantung kepada realiti, dalil-dalil, dan penerapan perkara-perkara tersebut ke atas fakta-fakta yang nyata. Aspek yang paling menonjol yang membezakan usul al-Shafi'i ialah bahawa ia mengandungi prinsip-prinsip umum bagi istinbat (penggalian) hukum, tanpa mengira apakah metodologi khusus beliau. Malah, usulnya sesuai dengan mana-mana metodologi, betapa pun berbezanya ia. Oleh itu, ia menjadi ukuran untuk mengetahui pendapat mana yang betul dan mana yang tidak betul. Ia ialah undang-undang yang menyeluruh yang mesti dipatuhi ketika menyimpulkan hukum baharu, apa pun metodologi yang ditetapkan oleh seseorang, untuk menilai pendapat-pendapat dan mengawal inferens hukum dengan satu undang-undang yang menyeluruh. Usul al-Shafi'i tidak dimaksudkan sebagai usul bagi mazhabnya sahaja, walaupun mazhab itu berpegang kepadanya. Ia tidak ditulis untuk mempertahankan mazhabnya dan menjelaskan pandangannya. Sebaliknya, ia mengandungi prinsip-prinsip umum dan menyeluruh bagi istinbat (inferens). Motifnya bukanlah kecenderungan kepada mazhab tertentu, tetapi keinginan untuk mengawal prosedur ijtihad serta menetapkan batasan dan garis panduan bagi para mujtahid. Beliau ikhlas dalam niatnya dan memiliki kefahaman yang betul ketika merangka ilmu usul al-fiqh, lalu mempengaruhi, tanpa kecuali, para mujtahid dan 'ulama yang datang selepas al-Shafi'i, sama ada mereka menentang atau menyokong pendapatnya. Sehingga, walaupun berbeza kecenderungan mereka, mereka mendapati diri mereka mengikuti jalan yang telah ditempuh oleh al-Shafi’i, dari segi menetapkan kaedah-kaedah kulliyyah (qawa’id kulliyya) dan melangkah dalam fiqh dan istinbat (inferens) secara teratur mengikut hukum-hukum yang menyeluruh dan prinsip-prinsip umum. Fiqh (kefahaman hukum Islam) selepas beliau menjadi berasaskan asas-asas yang mantap, bukan sebagai kumpulan fatwa dan keputusan individu (aqdiya) sebagaimana yang berlaku sebelum beliau. Walaupun semua ‘Ulama mengikuti jejak al-Shafi’i dari segi konsep usul al-fiqh, cara mereka menerima apa yang telah dicapai oleh al-Shafi’i berbeza mengikut pendekatan fiqh mereka yang berlainan. Sebahagian mengikuti pendapat beliau, mula menjelaskan dan mengembangkannya serta berbeza pendapat dengannya, seperti para pengikut al-Shafi’i sendiri. Dan sebahagian lain mengambil sebahagian besar daripada apa yang dibawa oleh al-Shafi’i, walaupun mereka berselisih pendapat dalam beberapa perincian usul, tetapi tidak dengan keseluruhan usul itu sendiri. Oleh kerana mereka tidak berselisih dari segi asas, kerangka dan haluan usul al-Shafi’i, seperti kaum Hanafi dan mereka yang mengikuti manhaj mereka. Dan ada yang berselisih dengan al-Shafi’i dalam usul ini, seperti kaum Zahiri dan Syi’ah. Mereka yang mengikuti al-Shafi’i dalam pendapat-pendapatnya ialah kaum Hanbali. Mereka menerima usul al-Shafi’i walaupun mereka berpendapat bahawa satu-satunya ijma’ (kesepakatan) yang diiktiraf ialah ijma’ para Sahabat. Kaum Maliki yang datang selepas al-Shafi’i menggabungkan metodologi mereka dengan sebahagian besar apa yang terdapat dalam usul al-Shafi’i, walaupun mereka menjadikan amalan penduduk Madinah sebagai hujah dan berbeza dengannya dalam beberapa perincian. Adapun mereka yang mengikuti manhajnya dan menerima pendapat-pendapatnya ialah para pengikut mazhabnya, yang sangat aktif dalam ilmu usul al-fiqh dan menulis banyak mengenai bidang ini. Kitab-kitab ditulis mengikut metodologi al-Shafi'i dalam usul al-fiqh, yang merupakan dan masih menjadi tiang serta penyokong bagi ilmu ini. Antara tiga buah kitab yang paling penting yang diketahui ditulis oleh ulama terdahulu: Pertama, kitab al-Mu'tamad karya Abu al-Husayn Muhammad ibn al-Basri (m. 413 H); kedua, kitab al-Burhan karya 'Abd al-Malik ibn 'Abd Allah al-Juwayni, yang lebih dikenali sebagai Imam al-Haramayn (m. 478 H); dan ketiga, kitab al-Mustasfa karya Abu Hamid al-Ghazali (m. 505 H). Selepas mereka muncullah Abu al-Husayn 'Ali yang juga dikenali sebagai al-Amidi. Beliau menghimpunkan ketiga-tiga kitab tersebut dan mengembangkannya dalam kitabnya al-Ihkam fi Usul al-Ahkam, yang merupakan salah satu karya paling penting yang ditulis dalam usul al-fiqh. Adapun mereka yang menerima sebahagian besar apa yang dibawa oleh al-Shafi'i tetapi berbeza pendapat dalam sebahagian perincian, mereka itu ialah golongan Hanafi. Ini kerana kaedah istinbat (pengambilan hukum) mereka selari dengan usul al-Shafi'i, namun cara mereka mendekati ilmu usul dipengaruhi oleh furu' (cabang-cabang fiqh). Mereka mempelajari prinsip-prinsip usul untuk menyokong furu', maka furu' dijadikan asas. Kaedah-kaedah umum dibina di atasnya dan dijadikan untuk menyokongnya. Barangkali yang mendorong mereka kepada pendekatan ini ialah bahawa kajian usul mereka bertujuan menyokong mazhab mereka, bukan untuk menghasilkan kaedah-kaedah yang dengannya mazhab mereka boleh mengeluarkan hukum. Ini kerana Abu Hanifah, yang mendahului al-Shafi'i, wafat pada tahun al-Shafi'i dilahirkan, dan istinbat beliau tidak berdasarkan kaedah-kaedah umum dan menyeluruh. Begitu juga selepas beliau, muncul murid-muridnya Abu Yusuf, Muhammad dan Zufar. Mereka tidak menumpukan perhatian kepada penulisan usul al-fiqh, tetapi kemudiannya tugas mengeluarkan kaedah-kaedah yang akan berkhidmat kepada furu' mazhab Hanafi jatuh kepada ulama mazhab Hanafi. Kaedah-kaedah itu muncul kemudian daripada furu' dan bukan mendahuluinya. Namun demikian, usul mazhab Hanafi secara keseluruhannya diambil daripada usul al-Shafi'i. Adapun perkara yang mereka berbeza pendapat dengan golongan Shafi'iyyah, iaitu tentang 'aam (umum) yang bersifat qat'i (pasti) sebagaimana khas (khusus)
What helped al-Shafi’i to lay down the science of usul was that he came at a time when Islamic jurisprudence had started to greatly flourish. In the Islamic lands juristical groups of mujtahidin began to take shape and they began to form into mazhabs (schools). The debate between the Mujtahidin and the proponents of mazahibs took various perspectives in fiqh and the evidences. So he plunged into debates with those who engaged in the debate, these discussions were what guided him to think about general and comprehensive principles as regulatory criterions which should be the basis of study and inference. He brought together these principles as one body of knowledge which was the science of usul al-fiqh. The impressive thing about the usul of al-Shafi’i is that he proceeds in the discussion of usul in a legislative and not in a logical manner. One of the greatest dangers for study, in fact for the Ummah’s revival especially in fiqh and usul is the path of logic. Al-Shafi’i clearly distanced himself from the course of logic and adhered to the legislative course. He was not interested in theoretical methods or suppositions. He wanted to regulate real and existing issues i.e, he took the Sharī’ah texts and stopped at the limit of the text and at the limit of the reality which the text indicated and the people themselves witnessed. Regarding the issue of abrogation (nasikh wal mansukh), he established the principles of abrogation from the issues which, for him, had been proven to contain abrogations, taken from what has been mentioned in the ayah or hadīth itself, or from the indication (dalala) of abrogation, or what has been narrated about the Messenger in terms of hadīth which indicate abrogation or whatever has been reported about the Companions of the Messenger of Allah in terms of reports and judgements. Not like many who came after him when they saw a contradiction between two verses or ahadīth they immediately moved to say that one has abrogated the other, to the extent that they ended up making terrible blunders. When al-Shafi’i came with a principle he did not bring it from a logical premise (muqaddima mantiqiyya) rather he showed the sources from which he took it, either from a report about the Prophet or from legal verdicts (fatwas) of the Sahabah. His approach in deriving regulatory (qawa’d dabita) principles was a practical one in which he relied on the reality, the evidences, and on the application of those things on tangible facts. The most prominent aspect by which Shafi’is usul is distinguished is that it contains general principles for the deduction (istinbat) of rules, regardless of what his specific methodology was. Rather, his usul is suitable for any methodology however different it may be. Thus, it is a measure by which one can know which opinions are correct and which are not correct. It is a comprehensive law which must be adhered to when deducing new rules, whatever methodology a person may set himself, in order to judge opinions and regulate the inference of rules by a comprehensive law. The usul of al-Shafi’i was not intended to be an usul for his mazhab (school) only, even though the mazhab adhered to it. It was not written to defend his mazhab and clarify its viewpoint. Rather, it contains general and comprehensive principles for istinbat (inference). The motive was not a trend towards a particular mazhab but rather it was a desire to regulate the procedures of Ijtihād and put in place limits and guidelines for the mujtahidin. He was sincere in his intentions and he had the correct understanding when devising the science of usul al-fiqh, thereby influencing, without exception, those mujtahidin and ‘Ulamā that came after al-Shafi’i, whether they opposed or supported his opinions. Until, despite their different tendencies, they saw themselves proceeding according to the path al-Shafi’i had taken, in terms of setting out comprehensive principles (qawa’id kulliyya) and proceeding in fiqh and istinbat (inference) in a regulated manner according to comprehensive laws and general principles. Fiqh (Islamic jurisprudence) after him came to be based on established foundations not as an assortment of fatwas and individual judgements (aqdiya) as was the case before him. Even though all of the ‘Ulamā proceed in the footsteps of al-Shafi’i in terms of the notion of usul al-fiqh, however the way in which they received what al-Shafi’ had arrived at was different according to their different juristic approaches. Some followed his opinions and began to explain and expand on them and disagree with them, like the followers of al-Shafi’i himself. And some took the major part of what al-Shafi’i had brought despite their disagreement with certain details of usul and but not the actual body of usul. Since they had no disagreements in terms of the body, framework and course of al-Shafi’is usul, like the Hanafis and those who followed their method. And there were those who disagreed with al-Shafi’i in this usul, like the Zahiris and Shi’a. Those who followed al-Shafi’i in his opinions were the Hanbalis. They adopted the usul of al-Shafi’i even though they said the only (recognised) ijma’ (consensus) is that of the Sahabah. The Malikis who came after al-Shafi’i combined their methodology with much of what was in al-Shafi’i’s usul though they took the practise of the people of Madina as a proof and differed with him in certain details. As for those who proceeded according to his method and embraced his opinions they are the followers of his mazhab who were very active in the (study of) the science of usul al-fiqh and wrote prolifically about the subject. Books were written according to the methodology of al-Shafi’i in usul al-fiqh which were, and still are, the pillars and support of this science. Of the most important three books that are known to be written by the ancients: First, the book al-Mu’tamad of Abu al Husayn Muhammad ibn al-Basri (d.413 A.H.), Second, the book al-Burhan of ‘Abd al-Malik ibn’ Abd Allah al-Juwayni commonly known as imam al-Harāmayn (d.478 A.H.) and third, the book al-Mustasfa of Abu Hamid al-Ghazali (d.505 A.H). After them came Abu al-Husayn ‘Ali otherwise known as al-Amidi. He brought together all three books and expanded on them in his book al-ihkam fi usul al-ahkām, which was one of the most important works written on usul al-fiqh, as for those who adopted the major part of what al-Shafi’i brought and differed in some of the details, they are the Hanafis. That is because their method of istinbat (inference) agreed with the usul of al-Shafi’i though the way in which they approached the science of usul was influenced by the furu’ (branches of fiqh). They studied the principles of usul in order to support the furu’. So they made the furu’ the basis. The general principles were based on it and made to support it. Perhaps what pushed them towards this approach was that their study of usul was for the purpose of supporting their mazhab and not in order to produce principles according to which their school should deduce rules. That is because Abu Hanifa who had preceded al-Shafi’i, died the year in which al-Shafi’i was born. And his inferences were not according to general and comprehensive principles. Likewise after him came his students Abu Yusuf, Muhammad and Zufar. They did not concern themselves with writing about usul al-fiqh but it fell to the scholars of the Hanafi mazhab afterwards to pursue the inference of principles which would serve the furu’ of the Hanafi mazhab, The principles came later than the furu’ and did not precede it. Nevertheless, the Hanafi usul on the whole has been extracted from the usul of al-Shafi’i. And what they differed on with the Shafi’is in terms of the ‘aamm (general) being qat’i (definite) like the khas (specific),