Praise be to Allah.
In linguistic terms, sharee‘ah refers to a water source, which is the source to which people who want to drink come, so they drink from it and fill their buckets, and perhaps bring their animals to drink from it as well.
The Arabs do not call a water source sharee‘ah unless the water is flowing without interruption, and is visible and can be seen.
Lisaan al-‘Arab (8/175).
In shar‘i terminology, sharee‘ah refers to the entire religion (Islam), which Allah has chosen for His slaves to bring them forth thereby from the depths of darkness into the light. It is what He has prescribed for them and what He has explained to them of commands and prohibitions, halaal and haraam.
Whoever follows the sharee‘ah of Allah, regarding as permissible (halaal) that which He has permitted and regarding as forbidden (haraam) that which He has prohibited, will triumph.
Whoever goes against the sharee‘ah (laws) of Allah has exposed himself to divine vengeance, wrath and punishment.
Allah, may He be exalted, says (interpretation of the meaning):
“Then We put you, [O Muhammad], on an ordained way [sharee‘ah] concerning the matter [of religion]; so follow it and do not follow the inclinations of those who do not know”
See also the answer to question no. 210742 .
In linguistic terms, fiqh means understanding. It may be said that so-and-so has been given fiqh fi’d-deen, meaning understanding of the religion. Another example is the du‘aa’ of the Prophet (blessings and peace of Allah be upon him) for Ibn ‘Abbaas: “Allahumma faqqihhu fi’d-deen (O Allah, give him understanding of the religion).” Narrated by al-Bukhaari (143) and Muslim (2477).
Tahdheeb al-Lughah (5/263).
In shar‘i terminology, fiqh refers to knowledge of the practical, minor shar‘i rulings which are derived from detailed evidence and proof.
Al-Mawsoo‘ah al-Fiqhiyyah (1/13).
Ibn Hazm (may Allah have mercy on him) said:
The definition of fiqh is: knowledge of the rulings of sharee‘ah as derived from the Qur’an and the words of the one who was sent with it (the Prophet (blessings and peace of Allah be upon him)), for these rulings can only be taken from him.
What is implied by this definition is: knowledge of the rulings of the Qur’an, and what abrogates and what is abrogated of it (an-naasikh wa’l-mansookh); and knowledge of the rulings in the hadiths of the Messenger of Allah (blessings and peace of Allah be upon him), what abrogates and what is abrogated of it, and what is soundly narrated of it and what is not; and knowledge of the matters concerning which there was consensus among the scholars and what they differed about; and knowledge of how to refer differences of opinion to the Qur’an and Sunnah of the Messenger (blessings and peace of Allah be upon him). This is what is meant by having knowledge of the rulings of sharee‘ah. End quote.
Al-Ihkaam fi Usool al-Ahkaam (5/127).
Ibn Jibreen (may Allah have mercy on him) said:
Fiqh is understanding of the texts, Qur’anic verses and hadiths, and knowing how to derive rulings from them.
Sharh Akhsar al-Mukhtasaraat (1/2)
With regard to usool al-fiqh,
The word asl (pl. usool) refers to the origin of a thing and what it is based on. Hence the father is the origin of the child, and the river is the origin of the channel.
Al-Misbaah al-Muneer (1/16).
Usool al-fiqh is knowledge of shar‘i evidence and the way in which that evidence is interpreted in order to reach a ruling, in general terms or in detail.
Sharh Mukhtasar ar-Rawdah (1/106).
Ibn ‘Uthaymeen (may Allah have mercy on him) said:
Usool al-Fiqh may be defined in two ways:
Firstly, on the basis of the two words that make up the phrase, namely the word usool and the word fiqh.
Usool is the plural of asl, which is the basis on which something else is built, such as the foundation of a wall (asl al-jidaar), or the trunk of a tree (asl ash-shajarah) from which the branches stem. Allah, may He be exalted, says (interpretation of the meaning):
“Have you not considered how Allah presents an example, [making] a good word like a good tree, whose root [asl] is firmly fixed and its branches [high] in the sky?” [Ibraaheem 14:24].
In linguistic terms, fiqh means understanding; in shar‘i terminology it refers to knowledge of the practical shar‘i rulings, with detailed evidence.
Knowledge may be definitive or speculative, because shar‘i rulings may be either definitive (yaqeeni) or speculative (zanni), as is the case in many issues of fiqh.
What we mean by the shar‘i rulings (conclusions) is the rulings that are learned from the shar‘i texts, such as the obligations and prohibitions. That excludes rulings (conclusions) derived by means of reasoning, such as knowing that the whole is greater than its parts, or regular rulings (conclusions), such as knowing that dew will come down on a winter’s night if the sky is clear.
What we mean by practical is that which does not have to do with beliefs, such as prayer and zakaah. That excludes anything that has to do with beliefs (‘aqeedah), such as the Oneness of Allah (Tawheed) and knowledge of the divine names and attributes. Such topics are not called fiqh in shar‘i terminology.
What we mean by detailed evidence is evidence of fiqh that has to do with detailed issues of fiqh. This excludes usool al-fiqh, because discussing issues of usool al-fiqh is done on the basis of the general evidence of fiqh.
Secondly, on the basis of the phrase (the two words together, usool al-fiqh) that forms the name of this branch of knowledge, it is defined as the branch of knowledge that deals with the general evidence of fiqh and how to make use of it, and the situation of the one who engages in this process (which is called ijtihad).
What we mean by general is the general principle, such as when the scholars said that a command means that something is obligatory, a prohibition means that something is haraam, and the soundness of a report means that it has authority. This excludes detailed evidence, which is not discussed within the framework of usool al-fiqh except by way of explaining a general rule.
What we mean by how to benefit from it is knowing how to derive rulings from the evidence by studying what the words of the text mean, and whether the meaning is general or specific, applicable with no restrictions or subject to some restrictions, what abrogates and what is abrogated, and so on. This is done by understanding all of the evidence of fiqh from which rulings are derived.
What we mean by the situation of the one who engages in this process is trying to find out his situation (and whether he is qualified), because he is the one who is engaging in this process (ijtihad), as he himself derives rulings through his efforts based on the evidence, as he has reached the level of ijtihad (having the ability to work out rulings based on the evidence). Knowing the definition of the mujtahid, the conditions and rulings on ijtihad, and so on, are matters that are discussed in the field of usool al-fiqh.
The benefit of usool al-fiqh is that it enables a person to attain the ability to derive shar‘i rulings from the evidence by means of a sound process.
The first one to write about usool al-fiqh as an independent branch of knowledge was Imam ash-Shaafa‘i, Muhammad ibn Idrees (may Allah have mercy on him); then he was followed in that by scholars who wrote various books on this topic. End quote. Al-Usool fi ‘Ilm al-Usool (p. 7-9).
See also the answer to question no. 220167 .
And Allah knows best.