The following is a modified version of a lecture recorded by Dr. Mohamed AbuTaleb and Sh. Omar Husain for Yaqeen’s Conviction Circles.™ You may watch the video here.

Introduction

Many of us are exposed to the Islamic sciences[1] through brief intensives, conferences, short online videos, weekend schools, and other similar venues. Each of these modes of learning has an important role to play in our education. However, there remains the real and pressing challenge of institutionalizing systematic learning true to the Islamic intellectual tradition and relevant to the modern context. Many Muslims memorize only a few short sūrahs of the Qur’an, seemingly satisfied with a surface-level understanding for the entirety of their adult lives and thus only experience most of the Qur’an vicariously through the commentary of others. Many miss an opportunity to build their own relationship with the text and lack a holistic view of its message. Many also lack a broad appreciation of the Prophet ﷺ or who he really was, relegated to knowing only an assortment of anecdotes without their corresponding context in his life, historical condition, or noble character. Without this foundational knowledge of the Qur’an and Sunnah, one is left vulnerable, particularly as one matriculates through higher levels of secular education without congruent maturity in understanding the Islamic worldview. This Islamic illiteracy results from these disjointed, disparate educational pursuits, often “more edutainment than education, [doing] little to rebuild the original Islamic culture of constant religious study throughout one’s life. This has led to a generation of young Muslims who are highly educated in almost every other field but possess a fifth-grade level of Islamic education.”[2]
In this paper, we introduce how the bases of the Islamic sciences—the uṣūl—serve not only as a source of academic regulation for scholars, but also as a gateway and a map for the everyday Muslim to understand the fabric of Islamic thought. The uṣūl afford the ability to see the world through an Islamic lens and to compare and contrast worldviews. Familiarity with the uṣūl offers a person a grounding to understand the big picture, to appreciate that the Islamic worldview forms a cohesive and interconnected whole. This is one important reason why the uṣūl may be more critical to our pursuits today than at any point in human history, given the unprecedented volume of information and pace of change experienced in the modern world. As much of contemporary Islamic pedagogy, andragogy, and discourse is preoccupied with what to think, we assert that Muslims must ground themselves in how to think, anchored by the uṣūl sciences.

What are the uṣūl?

At the outset of discourse on a topic, scholars of the Islamic sciences would often begin by defining terms. This helps establish a common terminology and identify the concepts necessary for the discussion at hand. Two complementary views on the usage of a term are commonly described. The first is the lexical definition: the meaning of the term in the classical Arabic language, outside of a technical Islamic paradigm. Classical poetry, prose, and the norms of speech originally formed these references. In following generations, dictionaries were compiled from these sources to record this knowledge. The same term can be used with more than one meaning, and usage context is critical to understanding the intended meaning of a word in any language. For example, consider the English term “surf”: without contextualizing information, we can not be certain if the speaker means surfing the waves on a beach or surfing the Internet.
The second dimension is the technical definition: the term’s usage by the scholars of a particular field of the Islamic sciences. The technical definition of a term is typically linked to the lexical meaning, as a related concept is applied to a new usage within the Islamic sciences. The uṣūl are a good example of these two complementary views of terminology.
The lexical definition of the term uṣūl is bases, essentials, or fundamentals: the foundations of something. Uṣūl is the plural Arabic term for the singular aṣl, meaning one basis or foundation. The technical definition when used within the Islamic sciences includes the rules and methodology governing a particular discipline as practiced by its scholars. Put differently, the term uṣūl is linked to a particular discipline that it is describing. For example, uṣūl al-fiqh means the foundations of Islamic jurisprudence, while uṣūl al-tafsīr means the foundations of Qur’anic interpretation. And when it’s used in this way, i.e., “uṣūl-something,” then it means the rules and methodology followed by the practitioners of that discipline: the process of regulation that helps govern the manner of research and reasoning within that discipline. 
Historically, some scholars have classified the uṣūl sciences as entirely separate sciences in their own right, due to their breadth and importance. Others considered them to be sub-disciplines or a part of the associated research discipline (e.g., jurisprudence). This is a technical difference in classification, helpful in observing the foundational place of the uṣūl in the Islamic sciences. 
Now, let us explore some examples of uṣūl sciences in the Islamic tradition. Uṣūl al-tafsīr, the methodology of Qur’anic interpretation, guides how the Qur’an is engaged, how interpretations are arrived at, and the overall rules and methodology that govern research and scholarly opinions in this discipline.
A second example is uṣūl al-ḥadīth, sometimes called muṣṭalaḥ al-ḥadīth, the methodology of hadith source criticism and evaluation. A hadith is a report attributed to the Prophet ﷺ regarding his statements, behaviors, and description. How do we evaluate the source of that claim, its chain of narration, the honesty and memory and details of every member in that chain of narration, and how do specialists ultimately arrive at an evaluation of the likelihood of the authenticity of this report? Is it a ṣaḥīḥ hadith, one that is highly authenticated, meaning that the Prophet ﷺ said it with a very high degree of likelihood? Is it ḍaʿīf (weak)? Is it ḥasan (good)? Is it mawḍūʿ (fabricated)? All of these matters and their associated details are part of the science of uṣūl al-ḥadīth.
A third important example is uṣūl al-fiqh, the methodology of Islamic jurisprudence, also known as Islamic legal theory. It encompasses not the rulings themselves, but how rulings are arrived at. There is a helpful analogy derived from the world of sports, worthy of careful consideration for its simplicity yet profound implications.[3] An understanding of this analogy can deepen one’s appreciation of how the respective uṣūl complement and relate to all of the Islamic sciences. Imagine any competitive sport, say soccer or football. In this game, there are athletes participating and ultimately the winner is decided by the score, even if many other statistics are tracked. 
In this analogy, we consider the athletes as the practitioners of the scholarly discipline, such as fiqh. (No analogy is intended comprehensively, and in this analogy the competitive aspect does not apply to the scholars, nor is the enterprise of scholarship a game). The athletes who are trying to score a goal are the specialists of fiqh who are seeking to offer a valid contribution that adds a new opinion, a helpful perspective, or a helpful analysis to the field. The players, in other words, are the jurists (fuqahāʾ): that is, the practitioners and specialists of fiqh. So who is the uṣūlī (i.e., the one practicing uṣūl al-fiqh) in this example? The uṣūlī in this analogy is the referee, who is not concerned with “scoring,” but is concerned with the proper application of the rules. In soccer, for example, you can’t grab the ball and score with your hand. That’s not a legitimate way to score; it violates the rules of the game. Similarly, in basketball, you can’t just run around without dribbling. It doesn’t matter if the ball goes through the hoop, as the play would not be considered legitimate—a traveling violation would be called and the basket would not count.
Analogously, in our Islamic disciplines, rules help govern what makes an appropriate contribution and the parameters for how to think. This protects these disciplines from otherwise becoming prone to anyone’s whims, desires, or baseless opinions that are introduced in the name of the religion. Ideally, it helps that a body of scholarship self-governs while being open to criticism and evaluation in a scholarly way. There is a balance between academic freedom and introducing new thought, while being tied to a consistent methodology of thinking. Without this methodology, Islam becomes anything to anyone. Any whim, thought, or opinion can be associated with the religion without a process of scholarly exchange, and a person can make a claim to the authority of the Qur’an or Islamic law or hadith without evidence. This is why the uṣūl are so important: when Islam becomes anything to anyone, then it becomes nothing at all. Thus, the uṣūl regulate the methodology of research and scholarship so that these bodies of Islamic sciences can be well-governed.

Were the uṣūl practiced at the time of the Prophet ﷺ?

Now we move to ask: were the uṣūl practiced at the time of the Prophet ﷺ? The answer for all of the uṣūl disciplines is yes. As long as there were rulings and scholarly opinions being offered, there were necessarily rules and methodologies regulating how those rulings came about. Let’s examine an example in the field of fiqh, or Islamic law and jurisprudence. 
At the time of the Prophet ﷺ, during his life and in his presence, there was no need for anyone to offer a scholarly ruling, because the Prophet ﷺ did not speak based on whims, but through revelation from God. Allah said, 
وَمَا يَنطِقُ عَنِ الْهَوَىٰ إِنْ هُوَ إِلَّا وَحْيٌ يُوحَىٰ

Nor does he speak from [his own] inclination. It is not but a revelation revealed.[4]

However, the Companions did make Islamic legal rulings during his lifetime, for one of two reasons. Either the Companions would be physically outside of the Prophet’s ﷺ presence such as on travel or in a different part of the city, and a situation would arise where they had to make a ruling; or the Prophet ﷺ would give them permission in his presence to offer their opinion on the matter. When this was presented to him, the Prophet ﷺ would either approve of the ruling, in which case it would become part of the Sunnah, and therefore part of the primary sources of our religion; or he would disapprove of it and correct it, and in that case that would also become part of his Sunnah ﷺ. So in that way, discerning the Prophetic methodology through primary source texts and through the rulings of the Companions, the science of uṣūl al-fiqh is derived.
The codification of these disciplines and the writing of books came after the generation of the Companions. This is not true just of Islamic disciplines, but is generally true of the fields of human enterprise, research, and scholarship. Writing down the rules of a field in books is not a prerequisite to the existence of a field—it’s part of the growth and maturity of that field. So, for example, in Arabic grammar and language, the Arabs during the time of the Prophet ﷺ didn’t learn Arabic like we do, from reading books and taking classes. They were masters of the Arabic language, able to gather in the city square and recite novel lines of advanced poetry extemporaneously. The language was learned as a living language through observation and osmosis. Their proficiency is unmatched by the overwhelming majority of people alive today; yet, they did not ever read a book on Arabic grammar nor sit in an Arabic language class.
Similarly, the uṣūl were part of that consciousness, but the fields were not codified and written down until later generations. As Islam spread to other lands, it started to become increasingly important to record and standardize discourse on these disciplines. There were many reasons for this. One reason is because Islam spread to more Arab and non-Arab lands, and with that naturally came a weakening of the Arabic language for both native Arabic speakers and for new Muslims coming into the religion who were not native Arabic speakers. Thus, it became very important to standardize and facilitate learning the language because later generations did not have the same sense of lexical meanings present in the Qur’an or in the hadith of the Prophet ﷺ. Similarly, as more and more people came into the religion, there were new situations and behaviors that arose that were never presented to the Prophet ﷺ or practiced by the Companions, so having the rules and regulations of uṣūl became more and more important. 
Ibn Khaldūn (d. 1406), the famous historian considered by many to be the father of modern sociology, writes in his masterpiece al-Muqaddimah that as these factors became more prominent and these enterprises (e.g., fiqh) became crafts of their own, the jurists of later generations came to need these rules and methodology in order to arrive at their rulings. It was then that the various disciplines started to glean and extract these timeless principles of the various uṣūl from the original sources. 

The foundations of the Islamic religion

There is one uṣūl science that is a notable exception in its usage of terminology. It uses the term uṣūl in its lexical meaning, literally meaning foundation, rather than its technical meaning discussed earlier. That science is the field of uṣūl al-dīn, the foundations of the Islamic religion. This discipline has been defined as “a science that enables a person to prove religious beliefs by stating appropriate evidence and by warding off misconceptions.”[5] There are important components of this discipline of uṣūl al-dīn, such as basic knowledge of the shahādah, the testimony of faith in Islam, and the ʿibādāt, the acts of worship. There’s also a component of this definition whose practical manifestation shifts with time: the part related to proving religious beliefs and warding off misconceptions, to avoid doubts (shubuhāt) and navigate these turbulent waters. As the surrounding context changes with time and place, the requisite knowledge considered part of uṣūl al-dīn changes as well.
Let us examine some concrete examples. An average person in our time has the potential to be exposed to more information in a single day than a person may have encountered in an entire year a few centuries ago, between social media, TV, globalized information exchange, and other sources. Many have experienced this information overload. As a consequence, a 21st-century Muslim in a globalized, interconnected world may need to know more about the foundations of their religion than a 17th-century farmer who knew and was comfortable with the essentials of Islam, since he was exposed to far less information. On the other hand, a graduate student, particularly in the liberal arts, may be in much greater need of the rationale behind sociological or legal rulings in Islam than an average Muslim, for whom these detailed rulings are not going to affect his daily life. Knowing these rulings will not inherently make him a better person: he will not make more ethical business decisions or grow in morality, and in case of need he could consult with a specialist. In contrast, a graduate student may have a vastly different experience because they may encounter this information day in and day out in academic discourse, and without knowledge of these disciplines, may become prone to misinformation or incorrect conclusions. 
Another example is a Muslim living as a minority in the West. There are certain rare circumstances or historical details that outside these circumstances, nobody but a specialist would need to know; yet, they are frequent targets of Islamophobic discourse. Examples are historical legal regulations on social constructs that no longer exist in society such as decisions on peace and war in pre-modern societies. Thus, it becomes important for Muslims to have basic knowledge of this, so they can ward off misconceptions that otherwise would have never intersected with their lives.
While this discipline may seem basic at first glance, mastery of it requires a very broad exposure to and deep understanding of the Holy Qur’an and the hadith of the Prophet ﷺ, in addition to a broad grasp of how Islam forms a cohesive, interconnected worldview. For this reason, major Islamic universities across the world house entire programs and colleges have been dedicated to the study of this one field for hundreds of years. We often see them called Kulliyat Uṣūl al-Dīn, or the College of the Foundations of the Religion. That is how important a mastery of this particular field of uṣūl continues to be to our day, both for individuals and for society as a whole. 

Ijtihād, reasoning, and evidence

We move now to a discussion of several important cornerstones of the uṣūl disciplines and particularly ijtihād, or legal reasoning. We begin with a hadith of the Prophet ﷺ, in which he spoke to Muʿādh ibn Jabal, the noble Companion he sent as an emissary to Yemen. In the absence of the Prophet ﷺ, Muʿādh would have to judge among people and issue rulings. This hadith gives us both the permission and the basic methodology of issuing Islamic legal rulings for qualified individuals. Al-Ḥārith ibn ʿAmr reported that the Messenger of Allah ﷺ sent Muʿādh to Yemen and he said, “How will you judge?” Muʿādh said, “I will judge according to the Book of Allah.” The Prophet said, “What if it is not in the Book of Allah?” Muʿādh said, “Then, with the Sunnah of the Messenger of Allah.” The Prophet said, “What if it is not in the Sunnah of the Messenger of Allah?” Mu’adh said, “Then, I will strive to form an opinion [lit. perform ijtihād].” The Prophet said, “All praise is due to Allah, who has made suitable the messenger of the Messenger of Allah.”[6]
This ijtihād, or scholarly effort and judgment, is part of what makes the religion suitable for different times, places, and contexts. The Qur’an contains broad, overarching values that are timeless and not restricted to a particular nation, gender, ethnicity, or time. Appropriate, bounded flexibility enables these values to guide human beings, whether riding a camel in 7th-century Arabia or a car in 21st-century America. This flexibility allows the jurist to engage the cultures and customs of people of different places and contexts under the common umbrella of timeless ethics, morals, and behaviors, while pursuing a holistic connection with Allah. Although these timeless principles are being interpreted by humans who are fallible, the field seeks to curtail errors or biases by governing scholarly efforts through an established methodology and ensuring that well-qualified people practice it. Moreover, the Prophet ﷺ taught us that his community, or ummah,  will not come to consensus upon misguidance. So if there is something that all of the scholars and all of the ummah of the Prophet ﷺ agree upon in scholarship, then it cannot be in error.[7] However, there remain differences of opinions in certain derivative areas of the faith which must be navigated with class and decorum. 
One of the ways that this field was regulated was by outlining a set of requirements for a specialist in the field. This may be familiar to many who are familiar with modern regulations concerning the practice of medicine, law, counseling, or other specialties. These fields are governed by regulations and requirements without which you cannot legally or ethically practice in that field, due to concern of harm that could befall the patient or client. In fact, much malpractice or unlicensed practice carries fines and punishments before an enforcement or government body. The scholars of the Islamic tradition have outlined various formulations of what it takes to be a mujtahid, or a practitioner and master of Islamic law. Today, many Islamic universities or training programs have captured this through prerequisites and degree requirements. For example, one such formulation outlines eight areas a person must master in order to be a mujtahid: 1) the holy Qur’an and its sciences; 2) the Sunnah; 3) the Arabic language; 4) the areas of scholarly consensus (ijmāʿ); 5) the methodology of Islamic jurisprudence (uṣūlal-fiqh); 6) the objectives of Islamic law (maqāṣid al-sharīʿah); 7) a familiarity with the conditions of people and the affairs of daily life (i.e., a person cannot rule on a situation they don’t understand or are not connected to); and 8) fairness and piety, being just in their rulings and of virtuous character and conduct.[8] All of these categories subdivide into further detail, as manifested in the academic departments and universities across the world. In short, these requirements represent a modern application of the Prophet ﷺ checking in with his emissary, the Companion, Muʿādh ibn Jabal, to help ensure that his application of the religion and its rulings would be appropriate.

Towards a classification of the Islamic disciplines of knowledge: How the uṣūl fit in with other Islamic sciences

As we’ve developed a basic understanding of the uṣūl disciplines and begun to understand their foundations, we move to ask how they fit in with the other disciplines of the Islamic sciences. Note that an understanding and appreciation of the uṣūl builds intellectual maturity and tolerance, as demonstrated throughout the history of Islamic scholarship. At the same time, history is messy. Intolerance or hurtful words may appear in the intellectual tradition, but so do truly awe-inspiring examples of tolerance and acceptance of other opinions. Such scholars do not equate religiosity with an adoption of their individual opinions, nor do they view their own scholarship as infallible. For example, among many other such examples, the famous Imām Sufyān al-Thawrī said,

“If you see someone doing an action upon which there is disagreement, and you have a different opinion than them, do not prevent them from what they are doing.”[9]

He states that when one sees something and is aware there is a difference of opinion on it, even if one thinks it’s wrong, he should not stop others or prevent them from their acts. 
Similarly, Imām Mālik ibn Anas, who was living in Medina and teaching in the Prophet’s ﷺ mosque, said that every one of us (i.e., all scholars) can refute and be refuted except for the occupant of this grave, at which point he pointed at the grave of the Prophet ﷺ. In other words, we can respect and hold our scholars in high esteem, but no one can claim infallibility or claim to speak on behalf of Allah except for the Messenger of Allah ﷺ. Everyone else, while according them due respect, is open to having their opinions challenged or being asked for evidence. This focus on evidentiary proof resulted in something remarkable for the Islamic intellectual tradition: it helped maintain cohesiveness of the religion and build upon the contributions of other scholars by contextualizing new opinions in an overall framework. To put it simply, a scholar should understand the tradition and build upon the contributions of earlier scholars, but he or she is neither trapped by the tradition, nor free to attribute any unregulated whim or opinion to Allah’s religion when it’s just coming from himself or herself without evidence. The power of evidence here is really hard to overstate. 
To examine the interconnectedness of the Islamic sciences, it is helpful to look at how scholars have classified these sciences in outlining their interdependence and relationships (i.e., how all of the sciences fit together). Many scholars have contributed to this across generations, and we offer a simplified summary of a common classification influenced by several works, including Ibn Khaldūn’s al-Muqaddimah. We examine a summary below to increase our appreciation of the uṣūl.
Consider the conceptual diagram above, in which a pyramid is split into three horizontal segments—a base, a middle, and a peak. At the base of the pyramid is the first classification of the Islamic sciences, called ʿulūm al-maṣādir, the source sciences. These disciplines are very clear in our religion: the Holy Qur’an and the Sunnah of the Prophet ﷺ. This knowledge is to swim in their oceans, to understand what’s there and what’s not there, what is emphasized and what is de-emphasized, and to have a broad exposure to the Qur’an and the words of the Prophet ﷺ. In fact, the notable tradition of many students of knowledge memorizing the entire Qur’an is indicative of this approach—not that memorization is an objective in and of itself, but it is a means to intimacy, closeness, and understanding the Qur’an that also sharpens and contextualizes subsequent studies. The scholar Ibn Abī al-ʿIzz states,

“The more fundamental and more needed by humanity a particular knowledge is, the more evident its proofs are—as a mercy from Allah to His creation.”[10]

In short, it is to know what is there and what is not. Many of us lack even this basic familiarity of the base of the pyramid. 
The second is ʿulūm al-wasāʾil, the gateway or tool sciences. These are the subclasses of disciplines that help a person understand the other disciplines of study. Notable among them is the Arabic language and its subcategories (e.g., syntax, morphology, rhetoric). Also within this middle category is what we’ve discussed here: the uṣūl disciplines (e.g., uṣūl al-tafsīr, uṣūl al-fiqh, uṣūl al-ḥadīth). This middle category helps us understand the others and understand what is a legitimate scholarly opinion and what is not, what follows a methodology and what does not, what builds on what has come before and makes a new contribution, and what ignores what has come before as if it doesn’t exist and merely asks a person to follow their independent platform and ignore others. These are all only possible through these uṣūl disciplines in the middle of the pyramid. 
At the peak of the pyramid are ʿulūm al-maqāṣid, or the objective sciences. This involves the application of what is beneath it. These disciplines include Islamic jurisprudence (fiqh), the interpretation of hadith, Qur’anic interpretation (tafsīr), behavior, character—all of these are applications and they are classified at the peak of the pyramid under the objective sciences.
Finally, the wrapping around this pyramid represents the fourth and final category: ʿulūm al-mutammimāt, or the completing sciences. This is like the icing on the cake—the things that beautify and embellish what you understand beneath them, such as the characteristics (shamāʾil) of the Prophet ﷺ (i.e., his physical and character description), the biographies of the Companions, the biographies of previous scholars, and so on. These beautiful stories of our tradition help settle these concepts in the heart and add beauty to these spiritual and intellectual pursuits.
Let us ask an uncomfortable, yet transformative question: how do our individual journeys of learning Islam map on to these categories?  Many of us are exposed to the Islamic sciences through short intensives, conferences, online reminders or short videos, well-intended but poorly organized weekend schools, and other similar venues. This is not to minimize the value of these pursuits, but to identify the real and pressing challenge of institutionalizing systematic learning true to the Islamic intellectual tradition and relevant to the modern context. Reflecting on their faith journeys in hindsight, many Muslims today find that instead of these brief touchpoints serving their intended purpose of reinforcement and reinvigoration of one’s faith, they actually form the spiritual and intellectual backbone of faith identity. Each of these modes of learning has an important role to play in our education. However, if we reflect on our journey in relation to this classification, we may realize that most of us began learning Islam exactly backwards with respect to this classification. 
We start with the short heart-softeners, those stories that package well in short 5-minute bursts and they’re beautiful, yet we don’t know what’s in the Qur’an. We’re stuck on auto-repeat on certain chapters; we don’t really know what's there—we’ve just heard others quote the Qur’an and talk about it. Many of us also don’t have a broad appreciation of the Prophet ﷺ or who he really was. Without this foundational knowledge of the Qur’an and Sunnah, we are left vulnerable, particularly as we progress through higher levels of worldly education. The uṣūl, therefore, are not only a source of regulation for scholars but they’re also a gateway and a map for the everyday Muslim to understand the fabric of Islamic thought: how to think Islamically and to compare and contrast worldviews. Familiarity with the uṣūl offers a person a grounding to understand the big picture, even if they don’t know a specific answer; when there is a question they don’t know the answer to, they retain knowledge of the system and structure that gave rise to other answers that they are more familiar and comfortable with. Contrary to many traditional programs of Islamic studies that emphasize this—especially for scholars, and including a basic and broad exposure for the everyday Muslim—it is not uncommon to find a Ph.D. student in Islamic studies, particularly in a Western university, who has had no exposure to the uṣūl disciplines whatsoever. This is at a time when perhaps the uṣūl are more important to our pursuits than at any point in human history. 
To deepen our appreciation of the uṣūl disciplines, we move to a specific examination of uṣūl al-fiqh at an introductory level in the next section.

The primary sources

Part 1: The Qur’an

We mentioned previously that uṣūl al-fiqh is the methodology of Islamic jurisprudence, also known as Islamic legal theory. It encompasses not the rulings themselves, but how the scholars deduce rulings. As one may assume, the first and most important source according to all scholars is the Qur’an, which contains many different types of verses. These include verses related to pondering the creation, devotional acts, dietary laws, social justice, family law, and many other topics. It is an all-encompassing Book. Along with these themes, we can further categorize verses to help us understand the difference between how an average Muslim versus a scholar of the religion approaches the Qur’an. 
Creed
One category of verses are those related to creed. For example, Allah says in Sūrat al-Ikhlāṣ, 
قُلْ هُوَ اللَّهُ أَحَدٌ

Say, 'Allah is One (Qur’an 112:1)

Any Muslim who reads this verse can understand the basic concept. Allah is One, and He has no partners. There is no dispute or difference of opinion on this idea. We also read the verse, for example, at the end of Surah Al-Baqarah:
آمَنَ الرَّسُولُ بِمَا أُنزِلَ إِلَيْهِ مِن رَّبِّهِ وَالْمُؤْمِنُونَ ۚكُلٌّ آمَنَ بِاللَّهِ وَمَلَائِكَتِهِ وَكُتُبِهِ وَرُسُلِهِ 

The Messenger has believed in what has been revealed to him from his Lord, and the believers as well. All have believed in Allah and His angels and His Books and His Messengers. (Qur’an 2:285)

As Muslims, we are required to believe in the angels, the books, and the messengers. Any Muslim who reads this verse is going to understand this basic concept. When the scholar reads the two aforementioned verses, however, their educational background will allow them to explore the verses in more depth. For instance, the scholar can refute the arguments of those who say there is more than one God. They can tell us what exactly angels are as a creation. They can answer questions such as how many angels are there? Who are they, and what are their responsibilities? What were the books of the previous nations that we are required to believe in? How many messengers were there? What exactly constitutes a messenger? The scholar will thus provide us with a thorough analysis regarding these verses. 
Character
Another category of verses are those related to character. For example, Allah says,
 وَقَضَىٰ رَبُّكَ أَلَّا تَعْبُدُوا إِلَّا إِيَّاهُ وَبِالْوَالِدَيْنِ إِحْسَانًا

Your Lord has decreed that you worship none but Him, and be the best to your parents. (Qur’an 17:23)

The average Muslim will read this verse and understand that they should worship Allah and treat their parents with exemplary character. The scholar will extract further meaning from the verse. They can tell us what  it means to worship Allah. What exactly are the rights that parents have over their children? When do children have to obey their parents, and when should they not obey their parents? These are matters of fiqh that require depth and proper training to understand. 
Ritual worship
The Qur’an also contains verses related to ritual worship, such as prayer, fasting, zakāh, and ḥajj. The average Muslim will understand these obligations at a basic level. The scholar will analyze these verses and answer questions such as: What is necessary to read in terms of the integrals of the prayer? What invalidates the prayer? What does not invalidate the prayer? What are the virtues of the prayer? What are the different narrations from the Prophet ﷺ on how he used to pray? Which of these narrations is the most authentic? This is the role of the scholar. What about the obligation to pay zakāh? Who knows how much to pay when we take into consideration something complicated such as stock markets? The scholar will explain and simplify some of the complex issues in zakāh for the average Muslim to understand. 
Transactions
The Qur’an is replete with verses related to transactions, such as marriage and divorce. The average Muslim can read these verses and develop some  understanding of marriage and divorce. The scholars will be able to research and understand the various issues surrounding marriage and divorce. These include questions such as what are the requirements of a valid marriage contract? What are the conditions for a dowry? What is considered a divorce? What are the prerequisites for a divorce to be valid? The scholars will assist in clarifying these often complex and life-altering decisions.

Part 2: The Sunnah

Why is the Sunnah a proof? This question is crucial to understand, because in our times there are people who want to deny the Sunnah. There are numerous verses in the Qur’an that talk about obeying the Prophet ﷺ. For example, Allah says,
قُلْ أَطِيعُوا اللَّهَ وَأَطِيعُوا الرَّسُولَ

Say, ‘Obey Allah and the Messenger.’ (Qur’an 24:54)

This is a direct command. How are we to obey the Messenger ﷺ? Through following his Sunnah. We also observe that in the lifetime of the Companions, they used to follow what the Prophet ﷺ ordered them to follow. They did not simply follow what they desired. While the Qur’an mentions certain obligations, it often does not provide the details pertaining to those obligations. If we are not following the Sunnah, then where are those details going to come from? It is important for us to understand that the Sunnah is part and parcel of our belief as well as a crucial source of uṣūl al-fiqh.
The Sunnah has been defined in various ways, depending on which Islamic science is being referenced. To keep it simple for our purposes, the Sunnah encompasses everything the Prophet ﷺ said, did, and approved of.
Sayings
An example of how the Prophet’s ﷺ sayings can play an authoritative role in defining law pertains to the permissibility of seafood. When asked about sea water, the Prophet ﷺ informed us that it is pure and its dead animals are permissible to eat.[11] This is different than, for example, if you find a dead animal on the side of the road. You cannot eat it because it is prohibited. Seafood, however, is different because the Prophet ﷺ explicitly allowed it. 
Actions
The next source consists of the actions of the Prophet ﷺ. This includes the obligations of prayer and ḥajj. Despite the fact that the Qur’an explicitly commands us to pray and perform ḥajj, it does not offer minute details on how they should be performed. This is where the actions of the Prophet ﷺ provide the legal precedents for ṣalah and ḥajj. 
Approval
Finally, there is the tacit approval of the Prophet ﷺ. For example, once two Companions came to the Prophet ﷺ for his advice on what had transpired between them. Earlier, they had been looking for water to make wuḍūʾ, and they could not find any, so they made dry ablution (tayammum).[12] Later on after finding water, one of the Companions repeated his prayer, while the other did not. The Prophet ﷺ gave his approval to both by not rebuking either of them. In other words, both were correct in the actions that they took. Thus, even in indirect ways, the Prophet established legal precedent. Recall the hadith mentioned earlier when the Prophet ﷺ asked Muʿādh what he would judge by, to which he said the Qur’an, and if he could not find it in the Qur’an, he would judge by the Sunnah.
Imām al-Shāfiʿī mentioned that the Sunnah embodies one of three categories[13]:
  1. The Sunnah emphasizes a Qur’anic demand. For example, Allah mentions prayer and ḥajj in the Qur’an. The Sunnah, in turn, emphasizes their rewards and importance.
  2. The Sunnah explains a Qur’anic concept. For example, how do we pray? The Qur’an commands us to pray, but does not mention all of the rituals, conditions, or integrals of the prayer. Similarly, the Qur’an commands us to fast. But what exactly does it mean to fast? What breaks the fast, what does not break the fast? We must turn to the Sunnah for many of these details. The same reasoning applies to zakāh and hajj.
  3. The Sunnah contains commands that are not mentioned in the Qur’an. An example of this is the prohibition of men wearing gold and silk.[14] This prohibition is not mentioned in the Qur’an; however, the Prophet ﷺ expressly forbade this in his Sunnah.
These are some of the ways that the Sunnah works in conjunction with the Qur’an. The Sunnah is a foundational concept in our faith, and cannot be dismissed.

The secondary sources

Part 3: Ijmāʿ

We turn now from the primary sources, the Qur’an and the Sunnah, to the secondary sources of uṣūl al-fiqh. The first secondary source is ijmāʿ. Ijmāʿ is the consensus of all mujtahid jurists in the same era on a particular ruling.[15] There is no difference of opinion that ijmāʿ is a part of uṣūl al-fiqh, but there are some differences as to its scope and possibility in later times. Some scholars maintain that it is more accurate to limit ijmāʿ to the Companions (ṣaḥābah), since knowing their opinions on issues was relatively easy. For example, we know that the Companions unanimously agreed to fight those who refused to pay the zakāh, and that they deemed it an obligation to settle the debts of the deceased before executing their will. An ijmāʿ was thus formed on these issues and they became legally binding. However, after the era of the Companions, the Muslim scholars were scattered in different lands, and their consensus was difficult to confirm. Nonetheless, the majority of scholars hold that ijmāʿ is binding and possible after the time of the Companions.[16]
What is the proof for ijmāʿ? There are many hadiths where the Prophet ﷺ indicates that the ummah will not gather upon misguidance.[17] Additionally, Allah states the following:
وَمَنْ يُشَاقِقِ الرَّسُولَ مِنْ بَعْدِ مَا تَبَيَّنَ لَهُ الْهُدَى وَيَتَّبِعْ غَيْرَ سَبِيلِ الْمُؤْمِنِينَ نُوَلِّهِ مَا تَوَلَّى وَنُصْلِهِ جَهَنَّمَ وَسَاءَتْ مَصِيرًا

Whoever breaks away from the Messenger after the right path has become clear to him, and follows what is not the way of the believers, We shall let him have what he chose, and We shall admit him to the hellfire, which is an evil place to return. (Qur’an 4:115)

The great exegete Ibn Kathīr mentions that this ayah was the proof that, after much thought and due consideration, Imam al-Shāfiʿī relied on to establish that ijmāʿ is a proof that cannot be opposed. He then mentions this is the best and strongest derivative deduction that ijmāʿ is legally binding.[18]
These are just some of the many texts that highlight the validity of ijmāʿ as a source of Islamic law. It is important to remember and clarify that ijmāʿ is not an independent source of law; it is dependent upon the fundamental sources. However, it is absolutely necessary in order to keep scholars from making erroneous or gross misinterpretations of certain core values and beliefs in the religion. Therefore, it serves as a protection of the religion. Ijmāʿ is particularly important in our times, because too often we have individuals making very bold claims that go against what has been unanimously agreed upon by the scholars of Islam.

Part 4: Al-qiyās

Another secondary source in uṣūl al-fiqh is known as al-qiyās. Al-qiyās is defined as “the deduction of legal prescriptions from the Qur’an or Sunnah by analogic reasoning.”[19] What is the proof for qiyās? In the hadith of Muʿādh mentioned earlier, the third source he said he would rule by if he could not find the answer in the Qur’an or Sunnah was ijtihād, which means to strive or struggle to find out something. Al-qiyās falls under the process of ijtihād. 
In another hadith[20], a woman came to the Prophet ﷺ and said “My mother vowed to go for ḥajj, but she died before she did so. Can I perform ḥajj on her behalf?” He ﷺ said: “Yes, perform ḥajj on her behalf. Do you not think if your mother owed a debt that you would pay it off for her? Fulfill her debt to Allah; for Allah is more deserving that what is owed to Him should be paid.” Put differently, the Prophet ﷺ analogized debt to ḥajj, and thus deduced a ruling on ḥajj. Just as she would be able to pay a debt for her mother, she could make ḥajj on her behalf.[21]
There are rules and regulations concerning qiyās, which consists of four essential elements:
  1. The Fundamental Issue (aṣl): One must first identify the fundamental issue. The example we will use is wine or alcohol. 
  2. The Secondary Issue (farʿ): Next, we identify a secondary issue that is not directly addressed in the Qur’an and Sunnah, but we need to determine its ruling. This secondary issue is known as the farʿ, which literally means branch. In our example, let us attempt to determine the ruling on heroin or cocaine, which is now our secondary issue.
  3. The Ruling on the Fundamental Issue: In order to determine the ruling on heroin or cocaine, we need to establish the ruling on the fundamental issue. When it comes to wine or alcohol, the ruling is that it is ḥarām or forbidden based on a clear Qur’anic commandment.
  4. The Effective Cause (ʿillah): Finally, the fourth prerequisite requires us to determine the effective cause, or the ʿillah, between our fundamental issue and the secondary issue. We can establish that both heroin and wine are detrimental to our state of mind. The reasoning behind prohibiting wine is because of its intoxicating effects. Thus, based on the original ruling that wine is ḥarām because of the ʿillah of intoxication, scholars rule that heroin and other mind-altering drugs are also ḥarām because of their intoxicating effects. 
One of the beauties of qiyās is that it shows us how universal our religion is. People may argue that there is no clear hadith that states that heroin is ḥarām, and is thus permissible. We can acknowledge that there is in fact no hadith that explicitly mentions heroin, but that is simply because this substance was not available at the time of the Prophet ﷺ. Thus, the ultimate purpose of uṣūl al-fiqh is to help preserve the religion and allow it to be relevant at all times. Therefore, when a new issue arises, we can use qiyās and make an analogical deduction to determine the ruling for a new issue. This holds true whether it is today, ten years ago, or far into the future.

Part 5: What about differences of opinion?

It is important to recognize that even the best of generations, the Companions of the Prophetﷺ, differed on some issues. The scholars have recorded many differences amongst the most illustrious Companions. One example is when two groups of the Companions were traveling to Banī Qurayẓah.[22] The Prophet ﷺ told them, “No one among you should pray ʿAsr except at Banu Qurayzah.” So, the companions set out, and the time for the ʿAsr prayer  commenced while they were still on the road. Some of the companions understood the Prophet’s ﷺ command to pray ʿAsr to emphasize they hasten to their destination of Banī Qurayẓah. Since they had been taught to pray every prayer during a fixed time, this group stopped and prayed on the road. The other group said, “We will not pray ʿAsr it until we reach Banī Qurayẓah.” This group understood the Prophet’s ﷺ statement literally, and did not stop to pray since that would be a direct contradiction of his explicit command, which is forbidden. Neither of the two groups were rebuked for what they did. When the Prophet ﷺ was informed of what happened, he did not say that either group was incorrect. They both made their judgment to the best of their abilities based on what they had learned from the Prophet ﷺ. 
It is also important to recognize that there are valid differences and invalid differences of opinion. The differences must remain within the uṣūl of Islam. To better understand this concept, let us take the example of two physicians. They both have the same training, but they may have different approaches as to how to treat a particular disease. That doesn’t necessarily mean that one is wrong or one is more correct—they simply have unique approaches. If one were to open a medical practice having just read some books or watched some videos on medicine, they would be prosecuted for medical fraud. Only after proper training and credentials will differences in their approaches be valid. Similarly, there have to be legitimate differences of opinion in the Islamic sciences. There are no differences in fundamental beliefs. No scholar, for example, will disagree that it is obligatory to believe in Heaven and Hell.
An example of a valid difference regards wiping the head in wuḍūʾ. The command to do so is based on the following Qur’anic verse,
يَا أَيُّهَا الَّذِينَ آمَنُوا إِذَا قُمْتُمْ إِلَى الصَّلَاةِ فَاغْسِلُوا وُجُوهَكُمْ وَأَيْدِيَكُمْ إِلَى الْمَرَافِقِ وَامْسَحُوا بِرُءُوسِكُمْ وَأَرْجُلَكُمْ إِلَى الْكَعْبَيْنِ

O you who have believed, when you rise to [perform] prayer, wash your faces and your forearms to the elbows and wipe over your heads and wash your feet to the ankles. (Qur’an 5:6)

This verse uses the Arabic letter bāʾ, which can imply wiping some of the head or all of the head. For this reason, some scholars deemed it obligatory to wipe only some of the head during wuḍūʾ, while others held that one must wipe the entire head. This is a legitimate difference of opinion based on the Qur’anic verse. In this case, it was the difference of a linguistic implication. Oftentimes, we view differences of opinions as contentious and perplexing. Many of these differences, however, add a level of richness, depth, and tolerance to our historic tradition, which is truly a blessing from Allah.
To summarize the discussion on legitimate differences of opinion, we conclude with the following quote from Dr. Nazir Khan: 

It is important, of course, to remember that our yardstick in addressing benefit or harm is not the yardstick handed to us by secular humanism, hedonistic materialism, or western liberalism. Rather, benefit and harm must be assessed according to the hierarchical value structure inherent in Islam, which situates human beings on a spiritual journey towards God as custodians upon His earth. 

With this, we have discussed some sources in uṣūl al-fiqh, and in shāʾ Allāh it is now clearer why we have differences of opinion and what room exists for valid differences of opinion in Islam.[23]

Notes

[1]  The Arabic term ʿilm (pl. ʿulūm), in its technical usage in identifying a field of knowledge, can be translated as “science,” “discipline,” or “knowledge.” The translations “science” and “discipline” are used interchangeably throughout this paper.
[2] Mikaeel Smith, “A Spiritual Disease in American Muslims Making Them Gods above God,” MuslimMatters, October 31, 2016, https://muslimmatters.org/2016/10/31/a-spiritual-disease-in-american-muslims-making-them-gods-above-god/.
[3] Suhaib Webb, “Monthly Worship: Special Opportunities Everyday,” (conference lecture, Beyond the 30: Making Every Moment Like Ramadan, Islamic Center of New England, Sharon, MA, July 16, 2011). 
[4] Qur’an 53:3–4.
[5] Muḥammad Ali al-Tahānawī, Mawsūʿat kashshāf iṣṭilāḥāt al-funūn wa al-ʿulūm, vol. 1 (Beirut: Maktabat Libnān Nāshirūn, 1996), 29.
[6] Sunan al-Tirmidhī, kitāb al-aḥkām, bāb mā jāʾ fī al-qāḍī kayfa yaqḍī, no. 1327. The hadith is narrated with similar wording in many sources, including Sunan al-Tirmidhī, no. 1328; Sunan Abī Dawūd, nos. 3592, 3593; among several others. Graded as sahīh (authentic) by Ibn ʿAbd al-Barr, Jamiʿ bayān al-ʿilm, vol. 2 (Dammam: Dār Ibn al-Jawzī, 1994), 844; and by Ibn al-Qayyim, Iʿlām al-muwaqiʿīn ʿan Rabb al-ʿĀlamīn, vol. 1 (Dammam: Dār Ibn al-Jawzī, 2002), 344–354.  See the latter for further detail on the chain of narration.
[7] See below for more information about ijmaʿ (scholarly consensus).
[8] Yūsuf al-Qaraḍāwī, al-Ijtihād fī al-sharī’ah al-Islāmīyah (Kuwait City: Dār al-Qalam, 1996), 17–51.
[9] Aḥmad ibn ʿAbd Allāh al-Aṣfahānī, Ḥilyat al-awliyāʾ wa-ṭabaqāt al-aṣfiyāʾ, vol. 7 (Cairo: Dār al-Fikr, 1996), 368.
[10] Ibn Abī al-ʿIzz, Sharḥ al-ʿAqīdah at-Taḥāwīyah, vol. 1 (Beirut: Muʾassassat al-Risālah, 1990), 38.
[11] Sunan al-Tirmidhī, no. 69.
[12] Sunan Abu Dawūd, no. 338.
[13] Imām al-Shāfiʿī, Kitāb al-risālah fī uṣūl al-fiqh (Beirut: Dār al-Kutub al ʿIlmīyah, 2019).
[14] Sunan al-Tirmidhī, no. 808.
[15] Al-Kūrāni, Al-darar al-lawamiʿ fe sharḥ jamʿ al-jawāmiʿ, (Madinah: Al-Jamiaʿh al-Islamiyah, 2008), 3:139.
[16] Abu al-Waleed Sulayman ibn Khalaf al-Bāji, al-Isharah fe maʿrifat al-Usūl wa al-wijāzah fe maʿna al-daleel (Beirut: Dar al Bashāir al-Islamiyah, 1996) , 280.
[17] Ibn ‘Umar narrated that the Prophet ﷺ said, “Indeed Allah will not gather my Ummah upon misguidance.” Sunan al-Tirmidhī, no. 2167. Though some of the scholars grade the isnād of this hadith to be weak, they maintain that the meaning is sound. Ibn Kathīr says, “There are many authentic hadiths with this meaning (i.e., that the ummah will not unanimously agree on something incorrect). In fact, some of the scholars have said that this meaning has been established by diffusely congruent (mutawātir) reports.” Tafsīr Ibn Kathīr (Riyadh: Dar Taybah, 1999), 2:412–13.
[18] Ibn Kathīr, Tafsīr Ibn Kathīr (Riyadh: Dār Ṭaybah, 1999), 2:413.
[19] The Oxford Dictionary of Islam, s.v. “Qiyās,” http://www.oxfordislamicstudies.com/article/opr/t125/e1936.
[20]] Saḥīḥ al-Bukhārī, no. 1754.
[21] It is important to note that making ḥajj on behalf of someone else is only permissible when that person is unable to do so themselves due to illness, death, etc.
[22] Saḥīḥ al-Bukhārī, no. 4119; Saḥīḥ Muslim, no. 1770.
[23] Nazir Khan, “Difference of Opinion: Where Do We Draw the Line?,” Yaqeen, December 10, 2019, updated June 22, 2020, https://yaqeeninstitute.org/nazir-khan/difference-of-opinion-where-do-we-draw-the-line.